harden: fail install verification on zero/short skill canonicals (#235)

[edheltzel]

Closes #235 (P1 from the #230 post-merge audit — reviewer report finding 1, the strongest-corroborated item in the set).

Problem

Since #228, the agent skills are the sole command surface. But recall_verify_install (lib/install-lib.sh) and recall doctor's buildSymlinkProbes (src/commands/doctor.ts) derived their expected-symlink list from whatever canonicals exist under ~/.agents/Recall/shared/skills/ with no lower bound. A bad npm pack, partial checkout, or an interrupt before _recall_copy_skill_files produces zero probes, missing stays empty, and install prints "All symlinks verified" over a blank surface. Pre-#230 the commands loop backstopped this; that backstop is gone.

Fix (surgical — a guard + tests, no refactor)

1. recall_verify_install asserts a floor: if $RECALL_REPO_DIR/agent-skills ships skill dirs, the canonical count under $RECALL_SHARED_SKILLS_DIR must match — else push a synthetic entry into missing so verification fails red. (canon < src, so partial packs are caught too.)
2. doctor: extracted listSkillCanonicalFiles() (shared with buildSymlinkProbes, DRY) + new exported path-injected probeSkillSurface() → WARN when the install root exists with zero skill canonicals; wired into runDoctor.
3. npm-pack.test.ts: assert all 9 agent-skills/<name>/SKILL.md are packed; assert commands/ is excluded.
4. install-sentinel.test.ts: Skills added to the unconditional-step guard; regression test for the empty-canonical floor.
5. doctor-skill-surface.test.ts: focused unit test for probeSkillSurface.
6. CHANGELOG [Unreleased] → Fixed.

Verification

• Red→green demonstrated for the floor guard: with the install-lib.sh hunk stashed, verify fails red when source ships skills but zero canonicals exist fails (recall_verify_install returns 0 — silent pass); with the fix restored it passes. Test pins the distinctive agent skill canonicals message so an unrelated missing symlink can't green it for the wrong reason.
• bash -n lib/install-lib.sh clean · bun run lint clean.
• Touched test files: 23 pass / 0 fail.
• Full bun test: 1235 pass. The 10 failures are pre-existing and unrelated — recall_configure_opencode_mcp / recall_configure_pi_mcp MCP-config tests (files not touched here); confirmed identical failures on the baseline with this branch's changes stashed.

Refs #230, #228. Do not merge — Themis gates.