fix(atomic): stop registering the statusline extension with atomic

[cameronraysmith]

Root cause

atomic 0.9.13 runs every interactive session's extensions inside an isolated RPC engine child, and that child's ctx.ui.setFooter is a warn-once no-op that neither binds its argument nor has a protocol frame to forward it.
The statusline extension calls setFooter at session start to evict atomic's native footer and put everything on one line, which is what it does under pi.
Under atomic the call is refused, so it prints ctx.ui.setFooter is unavailable in isolated interactive mode … on every startup, and — because atomic's footer dock is single-occupancy and only an honoured setFooter can vacate it — the native two-line footer stays docked beneath the extension's own status bar.
The extension's branch subscription and status pump live inside the same refused factory, so they never run either: that is why gate appears in atomic's native footer but never in the bar, and why the bar never repaints on a branch change.
The defect is upstream and atomic exposes no local remedy — no setting suppresses the native footer, and engine isolation cannot be disabled — so this change stops registering the extension with atomic while keeping it for pi.

Accepted trade-off

atomic loses the bar's subscription-usage and cost segments.
Everything else the bar showed is already in atomic's native footer: model, provider, cwd, branch, extension statuses, and context usage.
The consequence is that the two agents no longer render the same status line.
A shared single-line statusline across both is only reachable by fixing the refusal upstream, which this change does not attempt.

What changed

modules/home/ai/agent-settings.nix generates one settings payload for both pi-lineage agents, and packages becomes the single key they no longer share verbatim.
The divergence is declared there as its own named option rather than patched into a consumer, so one file still answers what each agent is given and why:

• atomicExtensionExclusions — the extensions atomic must not load, defaulting to statusline/index.ts, with the reason recorded beside it.
• packagesForAtomic — packages with those exclusions appended as --prefixed force-excludes to every entry that selects extensions.

atomic applies force-excludes last and unconditionally, so the - entry wins over the include of the same name and the shared list stays intact for pi.
The force-exclude form is preferred over deleting the include because it survives a later change that widens the include set, and because a pattern matching nothing is inert.

modules/home/ai/atomic/default.nix consumes packagesForAtomic.
It still declares packages rather than dropping the key: merge-settings.sh can update a nix-owned key but not retract one, so dropping the declaration would freeze whatever value last reached ~/.atomic/agent/settings.json.

New check

Nothing in the flake read programs.atomic at all, so this would have landed unguarded.
modules/checks/atomic-agent-environment.nix mirrors modules/checks/pi-agent-environment.nix, reusing mkStructuralCheck so a failure emits a unified diff naming the violated claim.

It asserts both halves of the divergence together, because either alone admits a wrong state: atomic must force-exclude statusline, and pi must still select it.
Asserting only atomic's half would pass a change that disarmed the extension for both agents.
atomicDeclaresPackages is asserted separately because the retraction failure above is invisible in the generated payload and shows only as the key's absence.

The check was verified severe rather than vacuous: emptying atomicExtensionExclusions fails it on atomicNegativeExtensions.

Verification

Evaluated settings, aarch64-darwin, crs58:

	statusline/index.ts	-statusline/index.ts
atomic	selected	force-excluded
pi	selected	absent

modules/home/ai/pi/ has a zero-line diff and pi's generated packages is byte-identical to main.

Runtime, against the settings this change generates, merged onto a scratch agent dir exactly as merge-settings.sh merges them — atomic --no-session --approve under a pty, rendered at 80x40:

                                                                0.0%/1.0M (auto)
────────────────────────────────────────────────────────────────────────────────
❯
────────────────────────────────────────────────────────────────────────────────
(zai) glm-5.3 high • ~/.treehouse/… (fm/vx-atomic-…)
 gate • ● ADHD Mode

Warning count 0; one footer rather than two; no blank gap; gate and ADHD Mode still present.
The runtime keys the merge must preserve — onboardedVersion, defaultModel, defaultProvider — survived.

Checks built locally on aarch64-darwin:

atomic-agent-environment-structural   (new)
pi-agent-environment-structural
pi-agent-environment-policy
pi-agent-environment-smoke
home-manager-crs58
home-configurations-exposed
home-module-exports
home-module-exports-neg
naming-conventions
eval-md-format

Buildbot covers the remaining checks and the non-darwin systems.

[mergify]

Tick the box to add this pull request to the merge queue (same as @mergifyio queue).

• Queue this pull request