fix(grok): expose advertised reasoning levels - #6386

Intent:
Keep Grok 4.6 usable from T3: plan-mode turns must stay reviewable over ACP,
advertised reasoning levels and user-invocable skills must show in the
composer, and the T3 runtime-mode control must actually drive Grok instead of
~/.grok/config.toml.

Behavior:
- Handle both x.ai/exit_plan_mode and _x.ai/exit_plan_mode, including wrapped
  payloads and null planContent.
- Emit the existing proposed-plan card from planContent, or fall back to the
  last session plan.md body when Grok races the plan-file write.
- Reply abandoned with a capture message so Grok unblocks without implementing
  in the same turn.
- Detect enter_plan_mode and promote writes to ~/.grok/sessions/.../plan.md
  onto the same card. Ignore workspace plan.md files, dedupe identical
  markdown per turn, and clear fallback state on settle or a new non-steer
  turn.
- Leave implement and request-changes to the existing proposed-plan follow-up;
  do not auto-approve plans even in full-access mode.
- Map each Grok model's ACP _meta.reasoningEfforts into the existing Reasoning
  select, preserving labels and descriptions and exposing the applied effort
  as currentValue. Keep one default badge and do not invent a menu when
  metadata is absent.
- Apply reasoning effort through session/set_model _meta.reasoningEffort,
  including effort-only changes. Do not carry an effort across model switches
  unless explicitly selected, skip set_model when nothing changes, and defer
  turn-time mutation until sendTurn validation succeeds.
- Honor the selected effort in Grok-backed title, commit, PR, and branch-name
  generation.
- Read user-invocable skills from grok inspect --json into the provider
  snapshot; use those names for both $ and /, skip bundled helpers, and leave
  both catalogs empty on inspect failure without failing provider discovery.
- Supervised spawns `--permission-mode default` so the thread asks even if the
  Grok CLI config is always-approve or auto.
- Auto-accept edits spawns `--permission-mode acceptEdits`.
- Auto spawns `--permission-mode auto` and sends session/new (and session/load)
  `_meta.autoMode: true`.
- Full access spawns `grok agent --always-approve stdio` and sends
  `_meta.yoloMode: true`.
- Probe and text-generation ACP processes omit a T3 mode and keep `grok agent
  stdio`.
- Always allow this session falls back to allow_once when Grok omits
  allow_always, then auto-approves later prompts in that session.
- Auto still escalates risky calls to T3; it is not always-approve.

Design constraints:
- Keep Grok-specific ACP, inspect, and permission dialects at the
  adapter/provider boundary. Do not change contracts or other providers.
- Reuse turn.proposed.completed, the existing plan card, and generic composer
  optionDescriptors / skills / slashCommands. Add no Grok-only approval
  prompt or new composer UI.
- Do not spawn-bind --reasoning-effort or require a new thread to change
  effort; Grok applies _meta in place.
- Do not map T3 Auto to yolo / bypassPermissions. That is Full access.
- Do not restart mid-thread on a runtime-mode change (separate hang: upstream
  pingdotgg#6517).

Integration:
- Grok adapter, xAI ACP extension helpers, ACP mock, ACP session runtime
  (set_model and session/new|/load _meta), Grok spawn args, Grok
  model/provider snapshot, inspect catalog parser, text generation, focused
  tests, internals provider docs, install.md, and permission-modes.md.
- Web, desktop, and mobile consume existing proposed-plan events, generic
  snapshot fields, and the existing runtime-mode picker with no client
  changes.

Verification:
- vp test run apps/server/src/provider/acp/GrokAcpSupport.test.ts
  apps/server/src/provider/Layers/GrokProvider.test.ts
  apps/server/src/provider/Drivers/GrokSkills.test.ts
  apps/server/src/provider/Layers/GrokAdapter.test.ts
  apps/server/src/provider/acp/XAiAcpExtension.test.ts
- Runtime-mode slice: 57 passed (GrokAcpSupport, GrokAdapter, GrokProvider).
- Earlier combined Grok slice: 64 passed including skills and xAI extension.

Rebase notes:
- Conflict hotspots are GrokAdapter plan handlers, sendTurn settlement,
  handleRequestPermission, buildGrokAcpSpawnInput, XAiAcpExtension.ts,
  GrokProvider discovered-model capabilities, GrokDriver inspect cwd, and
  AcpSessionRuntime setSessionModel plus session/new _meta.
- Upstream pingdotgg#4514 (plan), pingdotgg#5403/pingdotgg#6386/pingdotgg#6887 (reasoning), pingdotgg#4109 (skills),
  pingdotgg#6502/pingdotgg#6626 (Always allow and spawn permission-mode). Drop this patch only
  when main handles both plan-exit spellings with live/fallback plan capture,
  maps ACP reasoningEfforts and effort-only set_model, publishes grok inspect
  skills to $ and /, forwards all four T3 modes onto Grok argv with
  autoMode/yoloMode on session setup, Supervised overrides config.toml, and
  Always allow does not cancel when allow_always is missing.