Per-session enabledPlugins overlay (or COPILOT_ENABLED_PLUGINS env var)

### Problem

GitHub Copilot CLI reads `enabledPlugins` from a single global file at
`~/.copilot/settings.json`. Running two CLI sessions concurrently in
different repositories — a common case for engineers who keep multiple
project directories open — causes them to clobber each other's
`enabledPlugins`:

1. Session A starts in `/home/me/dev/projA`, the resolver writes
`enabledPlugins: { projA-plugin: true, projB-plugin: false }`.
2. Session B starts in `/home/me/dev/projB`, the resolver writes
`enabledPlugins: { projA-plugin: false, projB-plugin: true }`.
3. Both sessions function correctly because each loaded its plugins at
startup *before* the other session wrote.
4. The user runs `/restart` in Session A → the CLI re-reads
`settings.json` → loads `projB-plugin`, not `projA-plugin`.

This is structurally unavoidable from outside the CLI: there is no
per-session overlay, and `pluginDirectories` on the SDK
`session.create`/`session.resume` RPC (added in 1.0.55) is not exposed
to shell-launched `copilot` invocations.

### Proposal

Pick one of:

**Option A — Per-session settings overlay.** When the CLI knows its
`session_id`, additionally read
`~/.copilot/session-state//settings.json` (if present) and
merge it on top of the global `~/.copilot/settings.json`. Reconcilers
write the per-session file; the global file is never clobbered by
per-cwd activation decisions.

**Option B — Environment-variable override.** Honour
`COPILOT_ENABLED_PLUGINS=plugin-a@mkt,plugin-b@mkt` for the lifetime of
that invocation. The session's reconciler can compute the right set in
its launcher and pass it in, instead of writing to the global file.

**Option C — CLI flag.** `copilot --enabled-plugins=a@mkt,b@mkt`.
Same semantics as B but more discoverable.

Either A or (B + C) would let third-party tooling (e.g. workspace
managers, monorepo activation scripts) avoid the global-clobber
pattern.

### Why we want this

We ship a per-cwd plugin activator that picks a different plugin set
depending on which project the engineer is working in. It works fine
in single-session use but the cross-session clobber surfaces as
"my plugins disappeared after /restart" support tickets. We have a
warning-only mitigation in our reconciler, but there is no
deterministic fix from outside the CLI.

### Workaround today

We tell users to run all but the primary session with
`pluginResolver.mode: advisory` (a setting our reconciler reads, which
makes it log-only). This works but is bookkeeping the user shouldn't
need to do.

### Related

* Atomic writes for config/settings (shipped in 1.0.56) — solved torn
writes; the last-writer-wins semantics for `enabledPlugins` remain.
* `pluginDirectories` on `session.create`/`session.resume` RPC (1.0.55)
— solves it for SDK clients but not for shell users.

### Acceptance / how we would consume it

If A ships, we drop our reconciler's global writeback and write per
session. If B/C ships, we set the env var (or pass the flag) from our
session launcher. Either eliminates the race for our users.

Inizia tracciando il modo in cui il comando copilot avviato dalla shell legge ~/.copilot/settings.json e il modo in cui session.create/session.resume gestiscono pluginDirectories. Confronta gli approcci proposti dell’overlay session-state//settings.json, di COPILOT_ENABLED_PLUGINS e di --enabled-plugins con il flusso di configurazione esistente. Il lavoro è completato quando le sessioni concorrenti e /restart mantengono i propri enabledPlugins senza che una scrittura globale successiva li sovrascriva.

Scritto dal modello di indicizzazione a partire dal testo della issue.