fix: [ENG-2858] tool-mode dream CLI UX — finalize validation, SKILL.md, kinds drift-guard

Address findings from interactive e2e testing (blind + adversarial agents)
and /brv-pre-push-review:

brv dream finalize:
- Correct --help examples to match what scan emits: relative paths with
  the .html extension and hyphens (not underscores). Blind-agent tester
  copy-pasted the old style and hit a silent no-op.
- Reject combinations of --archive and --archive-file as mutually
  exclusive (previously --archive silently won, --archive-file dropped).
- Require at least one of --archive or --archive-file. A bare
  `dream finalize --session X` previously exited 0 with archived:[],
  which hides a typo'd flag in scripting.
- Cap archive batch at 200 entries per call. Without this, a multi-MB
  --archive string crashed the daemon with a raw Node stack trace
  (ERR_AGENT_DISCONNECTED). Callers with larger lists should split into
  multiple finalize calls.

brv dream scan:
- Derive VALID_KINDS from the canonical ALL_DREAM_KINDS on the daemon
  side instead of redeclaring the same literal array in the CLI. Prevents
  the two lists from drifting if a new kind is ever added.

SKILL.md:
- Add section "7. Dream — Context Tree Cleanup" documenting the
  three-phase scan → curate → finalize workflow, the four candidate
  kinds, undo semantics, and the v1 stateless-session caveat. Matches
  the existing pattern (Overview / Use this when / Do NOT use this when
  / Commands) used by the other command sections.