fix: address Copilot review round 1 on #2248

Five findings, all real.

**Critical — `verifySkills` selected `contents[0]`.** These bytes are hashed
against the file's advertised digest, so accepting a block the server
labelled something else verifies one file's content against another file's
digest, and can report that as `verified`. A false pass is worse than a
missing check. `contentsFor(result, uri)` now selects by NORMALIZED identity,
which keeps the canonicalized-echo case that motivated the original code
while rejecting an unrelated block, and reports a read error when nothing
answers for the URI. This is what `onReadSkillFile` already did; the two
paths hash bytes against digests and must not disagree about which bytes.

**`skills/get` was dispatched without the extension gate** that `skills/list`
had. Hoisted into `assertSkillsSupported` so the two cannot drift — declaring
the extension commits a server to both. Without it, an undeclared server's
-32601 is indistinguishable to a script from the -32602 a declared server
returns for a URI it does not serve.

**The TUI could strand a user on a hidden Skills tab.** The tab left the bar
when the gate went false but `activeTab` did not, so the pane kept rendering
for a server that never declared the extension. Reset to `info`, following
the Auth precedent — additionally gated on `connected`, because this gate
reads a server declaration and would otherwise fire during a reconnect.

**The TUI keyed a verdict by URI**, so a refresh replacing the manifest under
the same URI left hashes computed for the previous snapshot describing the
new one. Keyed on the entry now, as the web screen already was.

**A failed "Load more" discarded the pages on screen and the retry cursor.**
Both are preserved now, and the success and failure paths share one
staleness-guarded `commit` helper so they cannot drift on which results they
may write — which is how they came to disagree.

Two nits: the six-skill counts. The docs sentence was more wrong than
reported (five awkward skills of six, three outright violations) and is
rewritten rather than renumbered.

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>

docs/test-servers.md

@@ -88,10 +88,14 @@ It works on **either era**: `skills/list`, `skills/get` and
 era codec defines, so the SDK's era gate skips them entirely — which is why
 this fixture, unlike the tasks ones, needs no per-era variant.
 
-Four of the six skills are deliberately awkward, because the checks the Skills
-tab runs are untestable without them. Only two are actual violations — the
+Five of the six skills are deliberately awkward, because the checks the Skills
+tab runs are untestable without them. Only **three** are outright violations
+(`tampered-notes`, `lying-listing`, `wrong-folder` — the three `--verify` fails
+on). The other two are subtler and neither is an error on its own: the
 `"dynamic"` form is **conforming**, and is here because "legal but unverifiable"
-is the case most easily buried:
+is the case most easily buried; and `stale-manifest`'s entry is fully conforming
+too, with the defect living in the disagreement between its manifest and its
+directory listing:
 
 | Skill | What it exercises |
 | --- | --- |