docs: sync SEP copy with v1 and add AGENTS.md (#117)

* docs: sync repo SEP copy with canonical v1 text

The repo-local copy of SEP-2640 had drifted well behind the canonical
text on the sep/skills-extension branch, which was a source of
confusion: it still specified the well-known skill://index.json
resource and predated skills/list, skills/get,
resources/directory/read, nested skills, and per-file digests.

Replace the body with a verbatim copy of the canonical SEP as of
d7490ec (2026-07-15) and swap the stale "not actively maintained"
notice for a disclaimer that states what this file now is: a synced
snapshot, re-synced by overwriting rather than hand-editing, with
review on the upstream PR and anything beyond v1 raised as a PR in
this repository.

Also drop "working draft" from the README and related-work references
to this file, since it is no longer where the text is drafted.

* docs: frame the SEP copy as the v1 baseline for decision-log proposals

The previous note told readers not to propose changes here at all, which
is the wrong instruction: the WG does want to propose beyond v1, just in
this repo rather than on the upstream SEP.

Reframe the file as the v1 baseline and route proposals through the
existing decision log rather than a parallel mechanism. decisions.md
already carries the ADR-lite format, the Proposed/Accepted/Superseded
status idiom, and the precedent for consolidating a version's scope into
one record (the 2026-07-16 v1 entry), so a change beyond v1 is simply the
next dated entry. The note also defines the marker that ties an amended
section back to its record, so v1 text stays distinguishable from
accepted deltas as the document accumulates them.

Align the README and related-work references to the same framing.

* docs: mark the 2026-04-16 URI decision superseded in part

Every other decision the v1 scope-down changed carries a forward pointer
in its status line (2026-04-19, 2026-06-02, 2026-06-05, 2026-06-09). The
2026-04-16 URI entry was left as plain Accepted while three of its
bullets no longer hold, so it reads as current guidance for index.json
enumeration and the no-nesting rule.

Give it the same treatment, naming which bullets changed and which parts
of the URI convention carry forward, so the entry stays useful as a
record of the structure it settled.

* docs: add AGENTS.md and make the SEP PR link prominent

Readers were landing on the repo-local SEP copy and taking it for the
spec, which is how the stale version misled people in the first place.
Say plainly, in the places they land, that the v1 spec text lives on
modelcontextprotocol#2640:

- README: a source-of-truth line at the top of the link block, above
  the charter.
- The baseline copy: lead the header note with the PR rather than
  mentioning it mid-paragraph.
- AGENTS.md: state the rule for agents working in this repo, along with
  how to propose beyond v1 via the decision log and the repo's markdown
  conventions.

CLAUDE.md is a one-line pointer to AGENTS.md rather than a symlink, so
it resolves on Windows checkouts too, where symlinks need Developer Mode
and would otherwise land as a stub.

* docs: use the @AGENTS.md import in CLAUDE.md

A markdown link is not an import — Claude Code would have loaded the
pointer line and never expanded AGENTS.md into context. The documented
syntax is @path, which the docs specifically recommend over a symlink
on Windows, where symlinks need Administrator privileges or Developer
Mode.

The maintainer note is a block-level HTML comment, which Claude Code
strips before injecting the file, so it costs no context while staying
visible to anyone opening the file.

https://code.claude.com/docs/en/memory#agents-md

* docs: tighten AGENTS.md to the source-of-truth rule

Cut the project overview and conventions sections and lead with what an
agent most needs to get right: the v1 spec text is on the PR, and the
copy here is only as current as its pinned commit. CONTRIBUTING.md
already covers participation and process.

README.md

@@ -2,6 +2,7 @@
 
 > ⚠️ **Experimental** — This repository is an incubation space for the Skills Over MCP Working Group. Contents are exploratory and do not represent official MCP specifications or recommendations.
 
+**📄 SEP-2640 (Skills Extension):** [modelcontextprotocol#2640](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2640) — **the source of truth for the v1 spec text.** Review and comments belong on the PR. The copy in this repo is a synced baseline for discussion; see [`docs/sep-draft-skills-extension.md`](docs/sep-draft-skills-extension.md).
 **Charter:** [modelcontextprotocol.io/community/skills-over-mcp/charter](https://modelcontextprotocol.io/community/skills-over-mcp/charter) — mission, scope, membership, active work items, and success criteria.
 **Project board:** [Skills Over MCP WG](https://github.com/orgs/modelcontextprotocol/projects/38/views/1)
 **Meeting notes:** [Skills Over MCP WG discussions](https://github.com/modelcontextprotocol/modelcontextprotocol/discussions/categories/meeting-notes-skills-over-mcp-wg)
@@ -35,7 +36,7 @@ See [problem-statement.md](docs/problem-statement.md) for full details.
 | [Why Skills Over MCP?](docs/why-and-when.md) | Value proposition and decision guide |
 | [Use Cases](docs/use-cases.md) | Key use cases driving this work |
 | [Approaches](docs/approaches.md) | Approaches being explored (not mutually exclusive) |
-| [SEP-2640: Skills Extension](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2640) | Proposal to serve skills over MCP via the Resources primitive. Original working draft: [`docs/sep-draft-skills-extension.md`](docs/sep-draft-skills-extension.md). Comments and review happen on the PR. |
+| [SEP-2640: Skills Extension](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2640) | Proposal to serve skills over MCP via the Resources primitive. The v1 baseline text is kept at [`docs/sep-draft-skills-extension.md`](docs/sep-draft-skills-extension.md), synced from the canonical SEP. Comments and review of v1 happen on the PR; changes beyond v1 are proposed as entries in the [decision log](docs/decisions.md). |
 | [Open Questions](docs/open-questions.md) | Unresolved questions with community input (see also [issues](https://github.com/modelcontextprotocol/experimental-ext-skills/issues) and [meeting notes](https://github.com/modelcontextprotocol/modelcontextprotocol/discussions/categories/meeting-notes-skills-over-mcp-wg)) |
 | [Experimental Findings](docs/experimental-findings.md) | Results from implementations and testing |
 | [Related Work](docs/related-work.md) | SEPs, implementations, and external resources |