fix(cli): infer tasks for custom worktree paths and honor raw fallback fields

[dev-pi2pie]

Closes #23

Problem

gwtt list <task> -o raw could incorrectly fall back to the main worktree when the worktree path did not follow <repo>_<task> (for example, .claude/worktrees/<task>).
Root cause: classic-mode task matching relied only on TaskFromPath, so custom path layouts produced task "-" and no row match.

What Changed

• Added deriveClassicTask in cli/common.go:
  • First tries path-based extraction (TaskFromPath).
  • Falls back to branch-based task inference (SlugifyTask(branch)) for non-main, non-detached worktrees.
  • Avoids inferring a task for the main worktree.
• Updated classic-mode task resolution in:
  • cli/list.go
  • cli/status.go
• Fixed raw fallback output in list to respect --field:
  • path -> main worktree path
  • branch -> fallback branch
  • task -> -
• Added/expanded tests:
  • Unit tests for deriveClassicTask in cli/common_test.go
  • Integration coverage for custom path inference and raw fallback field behavior in cli/integration_test.go
• Updated docs:
  • README.md
  • docs/schemas/config-gwtt.md
  • Related plan/research/job docs for traceability

Verification

• go test ./... -count=1
• Added integration assertions for:
  • list and status task lookup with custom worktree paths
  • list -o raw fallback behavior with different --field values

Notes

• --branch remains the explicit/authoritative branch filter.
• Classic-mode task lookup is now path-first, then branch-backed fallback for eligible rows.