fix(plugin): isolate per-skill reattach failures + surface degraded plugins (#496)

[TYRMars]

Summary

Fixes #496.

PluginManager.#reattachOne iterated a plugin's skills with no per-skill error guard, and the MCP-server loop ran after it. On server restart, one unreadable or unparseable SKILL.md threw and aborted the rest of that plugin's reattach — every sibling skill and every MCP server the plugin declares. reattachInstalled then swallowed the rejection with a bare .catch(() => {}), and the package keeps no logger, so the failure was completely silent. Worse, GET /v1/plugins kept reporting the plugin as fully installed, because that response is backed by the ledger, not live state.

Changes

• Per-skill isolation (packages/plugin/src/manager.ts): each skill's reattach is wrapped in its own try/catch. A bad SKILL.md is recorded and skipped so it can no longer take out sibling skills or the MCP servers below it — matching SkillCatalog.scanRoot and the MCP loop's own best-effort intent.
• MCP failures recorded, not swallowed: the MCP add loop's failures are collected instead of vanishing into .catch(() => {}).
• Degradation is surfaced: added optional degraded / last_error fields to the InstalledPlugin ledger entry. They're recomputed on every reattach (cleared on a clean pass), and reattachInstalled also records a manifest read/parse failure onto the entry. Both fields flow through list() / get() to GET /v1/plugins (and the apps/jarvis-web client type), so a partially-reattached plugin stops reporting as fully healthy.

Testing

pnpm --filter @jarvis/plugin test (17/17) and typecheck pass; the changed files lint clean. New tests cover:

• a broken skill is skipped while its sibling skill and the plugin's MCP server both still reattach, with the entry marked degraded + last_error naming the bad skill;
• a failed MCP-server connect is surfaced as degraded without aborting the skill reattach;
• a stale degraded flag is cleared once the plugin becomes healthy again.

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https://claude.ai/code/session_01EHctPZgnTPnPvzQUHCWNzC

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