Version bump and full documentation pass for the v0.4 milestone.

Changes:
- Cargo.toml: 0.3.3 → 0.4.0 (release workflow reads this to tag and publish).
- CHANGELOG.md: convert [Unreleased] → [0.4.0] - 2026-05-04 with full
  Added/Changed/Fixed/CI sections covering all 16 commits in the milestone:
  Bitbucket Cloud forge (#240), Bitbucket Pipelines CI (#279), parsec
  compress (#236), ship --template (#233), ship --reviewer/--label and
  stack --submit (#261), stack navigation comments (#234), draft default
  (#238), [worktree] shared cache (#207), offline mode (#237),
  observability lite (#166), config JSON Schema (#239), Windows CI (#257)
  + UNC fix (#263), 11 integration tests (#278), release/** CI trigger
  (#277), and the docs cross-link update (#265).
- README.md: features list refreshed; new sections for parsec compress,
  parsec config schema, log --export JSONL, [worktree], [observability]
  config, Bitbucket env vars (PARSEC_BITBUCKET_TOKEN, BITBUCKET_TOKEN,
  PARSEC_BITBUCKET_API_BASE), PARSEC_OFFLINE; comparison table updated
  to 1st-class Bitbucket forge support.
- docs/index.html: 5 new feature cards (Bitbucket Cloud + Pipelines,
  Compress, Offline, Observability, Config Schema). Global options banner
  gains --offline.
- docs/reference/index.html: new compress command block, --offline in
  global options, log --export documented with JSONL example, config
  schema subcommand documented with schemastore reference.
- docs/guide/index.html: tracker.type values include bitbucket; new
  "What's New in v0.4" section walks through every v0.4 feature with
  end-to-end examples. Sidebar updated.

The release workflow will snapshot docs/{index,guide,reference}.html
into docs/v/0.4.0/ on merge — these updates ensure the v0.4.0 docs
snapshot is complete.

Cargo.lock is gitignored per repo convention; cargo build verified on
the bumped version locally.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>