feat(#3039): tier /gsd-help output (--brief, default, --full, <topic>) (#3040)

Replace the single 747-line /gsd-help reference with a progressive-disclosure
dispatcher (#2551 pattern). Newcomers get a one-page tour; returning users get
a 10-line refresher with --brief; the complete reference stays available behind
--full; /gsd-help <topic> emits one section; and /gsd-help --brief <topic>
is a compact scoped lookup (signature + one-line summary).

- workflows/help.md becomes a small dispatcher routing on $ARGUMENTS
- workflows/help/modes/{brief,default,full,topic}.md hold the tier bodies
- commands/gsd/help.md passes $ARGUMENTS through, advertises composable form
- docs/COMMANDS.md documents the new flags and topic form
- existing tests that read help.md repointed at help/modes/full.md
  (bug-2836, bug-2950, bug-2954, cursor-reviewer, execute-phase-wave)
- new feat-3039-help-tiered test enforces structure, size budgets,
  dispatcher routing, shim arg passthrough, topic→section coverage,
  orphan-heading detection, conflict-resolution rules, routing preamble,
  and compact-scope rule

Trek-e review fixes (PR #3040):
- topic.md output rules split into 5a/5b/5c — explicit handling for single
  sections, multi-section "plus" joins, and bold-line sub-block anchors;
  each rule also takes scope (full vs compact) into account
- explicit resolved-routing preamble line emitted by topic.md before content
  ("**Topic:** `<alias>` → `<heading>` *(scope: full | compact)*") so the
  user sees which alias matched at which scope (review finding #3)
- composable `--brief <topic>` invokes topic.md in compact scope: heading
  + first `**/gsd:*`** signature line + one-line summary. Dispatcher and
  topic.md cooperate via $ARGUMENTS pass-through (review finding #4)
- full.md capped at LARGE-tier budget (FULL_BUDGET = 1500); the non-recursive
  workflow-size-budget test does not reach modes/ subdirs
- structural <progressive_disclosure> table parse (5-row assertion) replaces
  substring-soup regex matching — 5 rows = 4 base tiers + composable scope
- forward /gsd:* sub-block token coverage + reverse orphan-heading allowlist
  catch alias-table drift in both directions
- four conflict-resolution tests guard dispatcher promises (--brief+--full
  without topic → --full; --brief <topic> → compact; --full <topic> or bare
  → full; dispatcher retains --brief when delegating to topic.md)
- hardcoded topic lists removed from docs/COMMANDS.md and full.md (drift
  surfaces reduced from 5 to 2)
- topic.md alias bloat trimmed (~75 → ~25 rows); cleanup/update split into
  distinct sub-block rows under ### Utility Commands
- comment-rot ("~750 lines") removed from default.md and full.md
- dispatcher size guard tightened from < 100 to <= 40 lines
- commands/gsd/help.md <process> block trimmed to one line
- MD040 fence languages added to all plain code blocks across mode files

Main-merge conflict resolution:
- workflows/help.md kept as dispatcher (body lives in help/modes/full.md)
- /gsd-<cmd> → /gsd:<cmd> rename from #3452 reapplied to the mode files
  (full.md, default.md, brief.md, topic.md) — the six namespace routers
  (/gsd-context, /gsd-ideate, /gsd-manage, /gsd-project, /gsd-quality,
  /gsd-workflow) and wildcards (/gsd-*) preserved in hyphen form per
  main's convention
- bug-2950 test combines branch's path repointing with main's namespaced
  replacement strings

docs/COMMANDS.md

@@ -548,12 +548,18 @@ Configure per-step flags in `.planning/config.json` under `manager.flags`. These
 
 ### `/gsd-help`
 
-Show all commands and usage guide.
+Show GSD commands at the tier you ask for. Default fits one screen; `--full` is the complete reference; `<topic>` jumps directly to one section.
 
 ```bash
-/gsd-help                           # Quick reference
+/gsd-help                           # One-page tour (default)
+/gsd-help --brief                   # ~10-line one-liner refresher of top commands
+/gsd-help --full                    # Complete reference (every command, every flag)
+/gsd-help <topic>                   # One section only (e.g. /gsd-help debug)
+/gsd-help --brief <topic>           # Compact scoped lookup — signature + one-line summary
 ```
 
+See `get-shit-done/workflows/help/modes/topic.md` for the full alias table. Unknown topics print the recognized list.
+
 ---
 
 ## Utility Commands