refactor(voyage): S31 — V15 trim plan-export to headless-only (decompose alias)

Drop the pr/issue/markdown variants from `/trekplan --export`. Claude Code
reformats a plan into a PR body, issue comment, or stripped markdown ad-hoc
on request, so a dedicated export path added maintenance without value.
Keep `--export headless` as a backwards-compatible alias for `--decompose`
and relabel it as the decomposition entry it actually is.

- commands/trekplan.md: Phase 1 parse rejects non-headless formats and sets
  mode = decompose for headless; delete the Export phase; rename Phase 1.6 →
  Phase 1.5 (Decompose); update the mode enum, argument-hint, and usage block.
- docs/command-modes.md, README.md: export row relabeled as a --decompose alias.
- tests/commands/trekplan.test.mjs: +2 V15 pins (variants gone, headless kept).

Non-breaking (plan D-register). Tests 731 (729 pass / 2 skip / 0 fail), bar
`node --test`; `claude plugin validate` green (1 accepted warning).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LqBYc8Ltrk7LipyJmGxXiB

README.md

@@ -236,7 +236,7 @@ Output:
 | **Foreground** | `/trekplan --project <dir> --fg` | No-op alias (foreground is default since v2.4.0) |
 | **Quick** | `/trekplan --project <dir> --quick` | No agent swarm, lightweight scan only |
 | **Decompose** | `/trekplan --decompose plan.md` | Split plan into headless session specs |
-| **Export** | `/trekplan --export pr plan.md` | PR description, issue comment, or clean markdown |
+| **Export** | `/trekplan --export headless plan.md` | Legacy alias for `--decompose` (the only remaining export format) |
 | **Profile** | `/trekplan --profile <name> --project <dir>` | (v4.1.0) Pin model profile; emitted as `profile:` in plan.md frontmatter. See [Profile system](#profile-system-v410). |
 
 `--brief` or `--project` is **required**. `/trekplan` with no brief exits with an error and a pointer to `/trekbrief`.