docs: link SEP-2640 across repo and tidy Related Work table (#90)

* Update documentation for Skills Extension SEP and related resources

- Revised links and descriptions in README.md to reflect the submitted Skills Extension SEP (PR #2640).
- Updated current status in approaches.md to indicate submission of the Skills Extension SEP.
- Clarified tracking information in open-questions.md regarding the Skills Extension SEP submission.
- Adjusted related work in related-work.md to include the Skills Extension SEP and mark a rejected proposal.
- Changed status in sep-draft-skills-extension.md to 'submitted' and updated author information.
- Enhanced skill-uri-scheme.md to indicate the incorporation of feedback into the submitted Skills Extension SEP.

* Move rejected SEP-2093 to the bottom of SEPs and Proposals table

Keeps the list active-proposals-first so readers aren't distracted by the
struck-out entry.

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* Mark SEP-2076 as closed and move to the bottom of proposals table

PR #2076 was closed without merge on 2026-02-24.

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* Use SEP-2640 naming convention per MCP SEP guidelines

Per https://modelcontextprotocol.io/community/sep-guidelines, a submitted
SEP is named "SEP-<PR-number>". Rename in-repo references from "PR #2640"
to "SEP-2640" and update the draft doc heading from the "SEP-0000"
placeholder to "SEP-2640".

Also mark Approach 1's SEP-2076 status as closed (2026-02-24, no sponsor
found); approaches.md still listed it as "Draft, seeking sponsor".

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* Fix SEP-2076 closure rationale

Closure followed the WG's decision to first explore a convention-based
approach using existing primitives, not a sponsor issue.

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* Refine SEP-2640 description in README to clarify proposal details

* Link issues and meeting notes in Open Questions README row

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* Surface SEP-2640 in README and related-work table order

README: move the Skills Extension SEP from the bottom of the Repository
Contents table to sit after Approaches, matching the narrative flow
(framing → options → proposal → supporting material).

related-work.md: lead the SEPs and Proposals table with SEP-2640 (this
WG's headline proposal), followed by the prerequisite PR #2527.

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Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* Add 'Open work' link to README header pointing to open PRs

Signals that alternative proposals, decisions, implementations, and
other in-flight contributions are welcome — SEP-2640 is one direction,
not the only one.

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/open-questions.md

@@ -121,7 +121,7 @@ See also [Approaches](approaches.md) for more notes on using resources.
 
 ## 13. What is the optimal relationship between skills and MCP?
 
-> **Tracked in:** [#75](https://github.com/modelcontextprotocol/experimental-ext-skills/issues/75) — Skills Extension SEP
+> **Tracked in:** [#75](https://github.com/modelcontextprotocol/experimental-ext-skills/issues/75) — Skills Extension SEP (submitted as [SEP-2640](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2640))
 > **See also:** [#47](https://github.com/modelcontextprotocol/experimental-ext-skills/issues/47) — Create evaluation matrix mapping approaches to requirements
 
 Skills already work as simple files that agents load directly. Adding MCP to the process should provide clear value beyond what standalone skills already offer.