Skills: Reload can't load a plugin installed after the chat started

Follow-up from AIR review on #94 (deferred there by decision).

**Problem.** A Claude plugin *installed after* a chat session is created cannot be loaded by the in-chat **Reload** — the Skills manager (which re-reads `installed_plugins.json` live) shows it, but Reload reports success while the running session still can't use it.

**Root cause.** `buildResourceLoader` computes `additionalSkillPaths` once at session construction. pi's `DefaultResourceLoader.reload()` re-derives its scan set from the private, construction-time `additionalSkillPaths` (it *overwrites* `lastSkillPaths`, so `extendResources` additions don't survive a reload), and `AgentSession` exposes no way to swap/reconfigure the resource loader. `#94` fixed the analogous case for **fixed-location** alias dirs (`.claude/skills` etc.) by pre-registering their candidate roots — but a plugin's install path isn't known ahead of time, and blind-scanning the plugin cache root is explicitly rejected (stale versions + transitive `node_modules/**/skills`).

**Options to explore.**
- A pi capability to reconfigure `additionalSkillPaths` (or swap the resource loader) on a live session, so the reload handler can re-read the plugin manifest and pick up new installs.
- Failing that, surface the limitation in the UI (Reload note: newly-installed plugins need a fresh session).

**Severity.** Real but narrow (plugin install mid-session); needs an upstream pi affordance to fix cleanly, which is why it was deferred out of #94.

Start with buildResourceLoader, the Reload handler, and the pi DefaultResourceLoader.reload and AgentSession behavior described here. Determine whether a live resource-loader reconfiguration can make a plugin installed after session creation available; otherwise verify the UI clearly notes that a fresh session is required. Done means Reload no longer reports success while the running session cannot use the newly installed plugin.

Written by the indexing model from the issue text.