feat: /gsd:fast command for trivial inline tasks (#609)

New lightweight command for tasks too small to justify planning overhead:
typo fixes, config changes, forgotten commits, simple additions.

- Runs inline in current context (no subagent spawn)
- No PLAN.md or SUMMARY.md generated
- Scope guard: redirects to /gsd:quick if task needs >3 file edits
- Atomic commit with conventional commit format
- Logs to STATE.md quick tasks table if present

Complements /gsd:quick (which spawns planner+executor) for cases where
the planning overhead exceeds the actual work.

Closes #609

get-shit-done/workflows/help.md

@@ -151,6 +151,21 @@ Usage: `/gsd:quick`
 Usage: `/gsd:quick --research --full`
 Result: Creates `.planning/quick/NNN-slug/PLAN.md`, `.planning/quick/NNN-slug/SUMMARY.md`
 
+---
+
+**`/gsd:fast [description]`**
+Execute a trivial task inline — no subagents, no planning files, no overhead.
+
+For tasks too small to justify planning: typo fixes, config changes, forgotten commits, simple additions. Runs in the current context, makes the change, commits, and logs to STATE.md.
+
+- No PLAN.md or SUMMARY.md created
+- No subagent spawned (runs inline)
+- ≤ 3 file edits — redirects to `/gsd:quick` if task is non-trivial
+- Atomic commit with conventional message
+
+Usage: `/gsd:fast "fix the typo in README"`
+Usage: `/gsd:fast "add .env to gitignore"`
+
 ### Roadmap Management
 
 **`/gsd:add-phase <description>`**