feat: Skills extension phase 3 — CLI, TUI, directory reads, frontmatter check

Closes #2248.

Completes SEP-2640 support across all three clients, and closes the one
obligation #2234 deliberately left open.

- `checkSkillFrontmatterMatch` compares a served SKILL.md's own YAML
  frontmatter against the entry the listing advertised, field by field. No
  digest can cover this: a digest is taken over the bytes the server served,
  so it proves the file was not altered in transit and says nothing about
  whether the listing described it honestly. Needs a real YAML parser; `yaml`
  was already a root dependency, so this adds no package — but it does newly
  put it on core/'s import graph, hence the three bundler `external` lists.

- `resources/directory/read`: result schemas defined against the normative
  text, `InspectorClient.readResourceDirectory`, a Directory section on the
  Skills screen, and `--method resources/directory/read` in the CLI. The
  client refuses the call locally when the server did not declare
  `directoryRead`, which is the SEP's MUST NOT. A child the directory lists
  but the entry does not is marked `not listed` rather than merged into the
  manifest — the SEP calls a directory result a live observation and forbids
  treating it as extending the manifest.

- CLI: `skills/list`, `skills/get` and `resources/directory/read`, plus
  `--verify` — one NDJSON report per skill, a summary on stderr, exit 7 on a
  violation. Reads are sequential; a conforming manifest may declare 512
  entries.

- TUI: a Skills pane, shown only when the server declares the extension. Each
  row carries its conformance verdict as a glyph as well as a colour, since
  the pane is read over ssh and through script(1). Enter verifies.

- `verifySkills` re-throws `AuthRecoveryRequiredError` rather than recording
  it per file: it says the session's authorization expired, so absorbing it
  would report N identical read failures and swallow the error the TUI and
  the web commands key off to reauthorize.

Two open questions settled, both in code comments where they will be found:
`skills/get` carries no caching attributes because SEP-2640 leaves the
question open in as many words, so requiring them would fail a conforming
server; and there is no `PagedSkillsState` because every consumer of this
list is a whole-catalog verdict — computed over page one of three, "this
server's skills conform" is not merely partial but wrong.

The fixture grows two skills, both for checks that were otherwise
undemonstrable: `lying-listing` (listing and file disagree, digest still
verifies) and `stale-manifest` (serves a file its manifest does not declare).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ND46Ci64DC3tzBy9aG81Nc
Signed-off-by: cliffhall <cliff@futurescale.com>

clients/tui/src/components/tabsConfig.ts

@@ -3,6 +3,7 @@ export type TabType =
   | "auth"
   | "resources"
   | "prompts"
+  | "skills"
   | "tools"
   | "messages"
   | "requests"
@@ -23,6 +24,11 @@ export const tabs: { id: TabType; label: string; accelerator: string }[] = [
   { id: "auth", label: "Auth", accelerator: "a" },
   { id: "resources", label: "Resources", accelerator: "r" },
   { id: "prompts", label: "Prompts", accelerator: "m" },
+  // `k`, not `s`: `s` is not in conflict today, but the accelerator has to
+  // appear in the label and be unique, and `S`kills against a future `S`ampling
+  // or `S`ettings is the collision this rule anticipates. `k` is the earliest
+  // remaining letter in the word after `s` and `i` (Info).
+  { id: "skills", label: "Skills", accelerator: "k" },
   { id: "tools", label: "Tools", accelerator: "t" },
   { id: "messages", label: "Protocol", accelerator: "p" },
   { id: "requests", label: "Network", accelerator: "n" },