fix(pi): declare the compaction extension Pi-only at the package level

[cameronraysmith]

Problem

atomic reads ~/.pi/agent/settings.json as a legacy config root, so it inherits every extension registered for pi.
One of them, pi-openai-server-compaction, imports @earendil-works/pi-coding-agent as a runtime value in src/remote-compaction.ts.
pi's jiti virtualModules map satisfies that specifier; atomic 0.9.13's does not, and the resolution failure is fatal on atomic's print and interactive paths:

Error: Failed to load extension ".../src/index.ts": Failed to load extension:
ResolveMessage: Cannot find module '@earendil-works/pi-coding-agent'

The other registered extensions survive only because their references to that specifier are import type, erased at transpile.

Fix

atomic's manifest reader is APP_NAME-first with a pi fallback (packages/coding-agent/src/core/package-manager-manifest.ts:24-27) and APP_NAME is "atomic", while pi reads only pkg.pi.
Injecting {atomic: {extensions: []}} into the package's package.json therefore short-circuits atomic to zero resources and is invisible to pi.

pkgs/by-name/pi-openai-server-compaction/package.nix becomes a stdenvNoCC.mkDerivation over the existing fetchFromGitHub, following the sibling pi-agent-extensions shape, and drops the -source suffix now that the output is no longer a bare fetch.
The ws runtime-dependency comment is preserved verbatim.

Constraints held

• The package remains a separate enabled entry and still appears in atomic list, satisfying the OpenSpec MUST at openspec/specs/pi-agent-environment/spec.md:74.
• Nothing is added to programs.pi-coding-agent.settings, so slowModeSettingsShape is untouched.
• compactionRetained derives both sides of its comparison from self'.packages.pi-openai-server-compaction, so it follows the new store path.

Verification

Ran the four checks that observe this derivation or its store path — the full set that would fail if the change were wrong. All green:

nix --accept-flake-config build .#checks.aarch64-darwin.package-pi-openai-server-compaction -L
nix --accept-flake-config build .#checks.aarch64-darwin.pi-agent-environment-structural -L
nix --accept-flake-config build .#checks.aarch64-darwin.pi-agent-environment-smoke -L
nix --accept-flake-config build .#checks.aarch64-darwin.home-manager-crs58 -L

pi-agent-environment-smoke is the real regression test: its fixture uses the live packageEntries and hard-fails on any extension_error.
It reported extension_errors=0.

Runtime behaviour was confirmed in an isolated HOME against scratch copies of the old and new outputs — never the captain's live config — with a marker line appended to the extension entrypoint:

host	extension output	result
atomic	old (negative control)	Cannot find module '@earendil-works/pi-coding-agent', exit 1
atomic	new	no extension error; reaches No API key found for the selected model.
pi	old	>>> VX_COMPACTION_EXTENSION_EVALUATED
pi	new	>>> VX_COMPACTION_EXTENSION_EVALUATED

The marker is positive proof that pi still evaluates the extension rather than merely not erroring.
atomic list was deliberately not used as an oracle: it renders this failure as a Warning: and exits 0, while the print and interactive paths render it as Error: and exit 1.

Deliberately out of scope

No atomic regulator was added to the checks suite.
The gap is real — nothing in the repo could have caught this, since pi-agent-environment-smoke is bound to mainProgram == "pi" and atomic's own --help test runs under an empty HOME — but adding one is a separate decision.
Also untouched: ATOMIC_CODING_AGENT_DIR handling, the pi module's settings, and the deleted node_modules output assertion.

Provenance

Diagnosis, fix selection, and the end-to-end verification with negative control come from the completed investigation at data/vx-atomic-pi-config-inheritance/report.md.

[mergify]

Queued — the merge queue status continues in this comment ↓.

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-08-17 01:02 UTC · Rule: default · triggered by rule queue human PRs
• ✅ Checks skipped · PR is already up-to-date
• ✅ Merged — 2026-08-17 01:03 UTC · at 534834c0bce14f77ba88828c2c43e6de2e27c949 · fast-forward

This pull request spent 39 seconds in the queue, including 6 seconds running CI.

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