feat: expand slash-command goals from command templates

Resolves armed slash-command objectives from authoritative templates before dispatch
Applies OpenCode argument expansion for goal metadata in UI and scheduled tasks
Falls back to the raw invocation when command details are unavailable

packages/web/server/lib/session-goal/DOCUMENTATION.md

@@ -160,6 +160,12 @@ sees only that final turn, so the report is its evidence.
   colors/labels shared across surfaces.
 - `stores/useSessionGoalArmStore.ts` — the "next prompt starts a goal" flag,
   consumed by `sendMessage` in `sync/session-ui-store.ts` (works for drafts).
+  Armed slash commands resolve their authoritative command template and apply
+  OpenCode argument expansion (`$ARGUMENTS`, positional placeholders, or the
+  implicit argument suffix) for the audit objective before goal metadata is
+  written and before `session.command` dispatch. If command details cannot be
+  loaded, the raw invocation remains the objective rather than blocking command
+  execution.
 - `hooks/useSessionGoal.ts` — live goal state.
 - `components/chat/SessionGoalButton.tsx` — composer target button
   (arm / status color / cancel confirm); `SessionGoalRow.tsx` — goal strip
@@ -172,7 +178,8 @@ sees only that final turn, so the report is its evidence.
 Scheduled tasks can run as goals: `execution.goalEnabled` (+ optional
 `execution.goalTokenBudget`) on a task makes the scheduled-tasks runtime
 stamp `metadata.openchamber.goal` onto the fresh session (objective = the
-expanded task prompt) and attach the goal-mode intro part to the prompt.
+expanded task prompt, or the argument-expanded command template for a slash
+command) and attach the goal-mode intro part to normal prompts.
 The loop here picks it up from session events like any other goal.
 
 ## CLI-created goals
@@ -183,8 +190,10 @@ session, fits and stores the expanded prompt as its objective, patches active
 goal metadata, appends the synthetic goal reminder, and only then dispatches
 the prompt. `--goal-token-budget` applies the same optional budget contract as
 scheduled goals. Slash commands retain command dispatch semantics and cannot
-carry the synthetic prompt part; the goal metadata is still installed before
-the command runs.
+carry the synthetic prompt part. Their command template with OpenCode argument
+expansion becomes the audit objective; goal metadata
+is still installed before the command runs. A missing command template falls
+back to the raw invocation.
 
 `openchamber session send --goal` and `openchamber session fork --goal` use
 the same server-owned prompt orchestration. Send installs a fresh goal on the