fix(cli): describe exact leaf command paths

[rrrrrredy]

Summary

command describe now resolves the complete registered command path. Describing
a leaf returns that leaf's canonical path and metadata without executing its
capability.

Why

• Issue/Story: Fixes command describe ignores leaf command paths #33
• User or maintainer outcome: CLI consumers can inspect an exact leaf command
  instead of silently receiving metadata for its parent.

Traceability and Scope

• Spec/ADR, if applicable:
  docs/specs/2026-07-31-33-describe-leaf-command-paths.md
• Acceptance criteria addressed: AC-1 through AC-5
• Canonical owners changed: scripts/better-harness-cli/registry.mjs and
  scripts/better-harness-cli/cli.mjs
• Explicit non-goals: command dispatch, registry contents, OpenCLI schema,
  capability-owned scripts, and global option parsing

Change Type

• Feature
• Bug fix
• Tests only
• Documentation/community
• Refactor with no intended behavior change
• Dependency, packaging, or infrastructure

Test and Review Evidence

Check	Result
node --test --test-concurrency=1 test/better-harness-cli.test.mjs test/scripts-refactor-contract.test.mjs	39 passed, 0 failed, 1 Windows symlink-permission skip
node scripts/doc-link-graph/cli.mjs skills/better-harness	34 files and 50 links
node --test test/doc-link-graph.test.mjs	6 passed, 0 failed
npm run pack:verify	Passed: 359 npm entries and 382 runtime-zip entries
node --test	1015 passed, 10 failed, 3 skipped; all failures were Windows EBUSY cleanup or EPERM symlink host errors
Same-name checks on untouched origin/main	Reproduced the same EBUSY/EPERM failure classes; no changed CLI assertion failed
git diff --check	Passed

Manual or visual evidence:

• command describe harness render --json returns name: "render",
  path: ["harness", "render"], and the registered render script.
• Human output identifies Command: harness render.
• Unknown leaves return UNKNOWN_SUBCOMMAND; third path segments return
  INVALID_COMMAND_PATH.

Risk and Recovery

• Compatibility and cross-platform impact: parent descriptions and aliases are
  unchanged; leaf JSON gains an additive canonical path field.
• Package, plugin, schema, or generated-file impact: no schema or package
  boundary change; the frozen root-help fixture and changelog are updated.
• Rollback or recovery path: revert this commit to restore parent-only
  description behavior.
• Residual risk or unverified boundary: local Windows symlink tests require
  privileges unavailable on this host; CI remains the cross-platform gate.

AI Involvement

• Level: Assisted
• Human review and validation: I defined the expected behavior and reviewed the
  focused, full, documentation, and package validation results before submission.

Checklist

• I followed AGENTS.md, CONTRIBUTING.md, and the relevant canonical-owner guidance.
• The change is focused and does not include unrelated local or generated state.
• Tests and documentation match the behavior actually delivered.
• Markdown links were checked when documentation moved or changed.
• Cross-platform behavior was considered for Windows, macOS, and Linux.
• Package/runtime verification was run when shipped files or dependencies changed.
• User-facing or compatibility changes are recorded in CHANGELOG.md.
• I have the right to contribute this work under the repository's MIT License.