coding-agents: add pi as a harness, and give Prime Agent the companion skill (#3772)

[feniix]

Closes #3772.

Prime Agent discovers ~/.prime/agent/skills and the shared ~/.agents/skills exactly as pi discovers its own, but nothing installed a skill for it — so it only ever picked ours up by accident, from the shared root, when the user also ran Codex or dsh.

The change for #3772

const primeAgent = piFamilyInstaller(
  "prime-agent",
  [".prime", "agent"],
  [".prime", "agent", "skills"],   // <- was missing
);

Into its own root, not the shared one: uninstallSkill removes by a fixed directory name, so writing to ~/.agents/skills would make uninstall prime-agent delete Codex's and dsh's copy. There's a test for that.

With both hosts passing one, skillsDir stops being optional — no caller omits it now — so the parameter and its two if (skillsBase) guards become required and unconditional, and the uninstall log line stops under-reporting (it removes a skill as well as the extension entry).

The test that pinned prime-agent as deliberately skill-less is replaced by the pi cases generalised over both hosts, plus one asserting the two roots stay independent: uninstalling pi must not take Prime Agent's skill with it.

Why this PR carries more than that one-liner

piFamilyInstaller does not exist on main — it arrives with the pi harness work in the first five commits, which are not yet on their own PR. #3772's fix is one argument to a factory that this branch introduces, so the two cannot be separated without rewriting the fix against the older hand-written primeAgent installer block.

Read it as the pi harness, then #3772 on top. The last commit (a5fd6f17e) is the fix the issue asks for; the rest is its prerequisite:

commit	what
dadc31aa7	refactor: extract the pi extension adapter Prime Agent shares — no behaviour change
a19b0bde4	feat: add pi as a supported harness
776f53218	feat: import past pi and Prime Agent sessions from local disk
fde3bb331	fix: don't let one stray file abort the pi history import
7ab072245	feat: give pi the companion skill, and guard every entrypoint's harness name
a5fd6f17e	feat: give Prime Agent the companion skill too — #3772

Each commit builds and passes on its own (verified by checking each out in a worktree).

Background on the pi commits

pi (@earendil-works/pi-coding-agent) is the agent Prime Agent was forked from, and the two expose the identical extension API — same extensions array in settings.json, same before_agent_start / agent_end events, same registerTool, same pkg.pi manifest key, same stored-session schema. So the existing Prime Agent adapter fits pi unchanged; it just had no seam to plug into. The refactor adds one (createPiExtension(harness), mirroring createPluginEntry for the opencode family), and pi becomes a thin entrypoint alongside Prime Agent's.

Both hosts still share one bank per repo — the default coding-agent::{gitProject} is harness-neutral on purpose — so a separate entrypoint per host is about attribution (harnesses.<name> config section, metadata.harness on retained documents, control-plane logo), not about splitting memory.

Two notes for reviewers:

• The pi manifest key stays pointed at Prime Agent's bundle. Both hosts read the same pkg.pi.extensions, so it can only ever name one. The README tells pi users to run install pi rather than pi install npm:…, which would otherwise report pi's sessions as prime-agent.
• claudeHistory has the same ENOTDIR exposure fde3bb331 fixes for pi and is deliberately untouched here — filed separately as coding-agents: --import-conversations dies on a stray file in ~/.claude/projects instead of skipping it #3771.

Verification

• 688 tests pass; tsc, ./scripts/hooks/lint.sh and ./scripts/hooks/check-unused.sh clean.
• Both regression tests were confirmed to fail without their fix (ENOTDIR for the history guard; expected 'prime-agent' to be 'pi' for the entrypoint guard).
• History readers validated against real stores: 202 pi sessions attributed to one repo with zero unattributed, in 760ms across a 34k-folder tree.
• Generated files (skill/SKILL.md, the docs page, the docs-skill mirror) regenerated per commit, so each one is self-consistent.

[strix-security]

Strix Security Review

Warning

This pull request has 17 commits after the last Strix review (a5fd6f1). Strix has not reviewed these changes.
Automatic review on push is off for this repository. To review the latest changes, tag @strix-security in a comment, or turn on re-review on push.

No security issues found.

_{Updated for a5fd6f1.}

---

Reviewed by Strix
_{Re-run review · Configure security review settings}