fix(skills): show every engine in the /skill list matrix

[clawedassistant26]

/skill list prints the support matrix for every engine moshcode wraps. It is currently missing one.

The bug

printSkillTargets (src/integrations.mjs:109) prints the supported engines from SKILL_ENGINES, then the unsupported ones from a hardcoded ["codex", "opencode", "aider"]. ENGINES has six keys. Since privacycode was added (#74) it is in neither list, so it gets no row at all.

On main (df9d1e0), /skill list prints:

  skills  — install a skill everywhere with /skill install <git-url>
   ○ claude    skills supported
   ○ gemini    skills supported
   ○ codex     no skills primitive
   ○ opencode  no skills primitive
   ○ aider     no skills primitive

Five rows for six engines. privacycode is installable, has an alias (pc), and appears in /mcp list — but a user checking whether /skill install reaches it sees nothing, and cannot tell an unsupported engine from a name they typed wrong.

The fix

Derive the unsupported rows from Object.keys(ENGINES) minus SKILL_ENGINES. Four insertions, three of them the comment. privacycode is an opencode derivative so it genuinely has no skills primitive — the point is that it now says so instead of vanishing.

This also means the matrix cannot drift from the engine list again: the next engine added to ENGINES shows up automatically.

Tests

New test/support-matrix.test.mjs, 5 tests:

• 3 cover the bug: every engine appears in /skill list exactly once; privacycode is marked no skills primitive; each row's label matches its SKILL_ENGINES membership.
• 2 are controls over /mcp list, which pass both before and after this change. MCP_ENGINES already covers everything except the hardcoded aider row, so the MCP matrix is complete today. I left printMcpTargets alone and used it as proof that the change is scoped to the skills path.

Fail-before / pass-after (verified by restoring pristine integrations.mjs, not git stash): 3 fail / 2 pass unpatched, 5/5 patched.

Full suite: root npm test 204 → 209, 0 fail. apps/pwa 30/30, unchanged.