Session-only model selection for Agent Sessions lanes

[antst]

• Queued (not yet posted): DSH Discussion: session-only model selection, restated for the Agent Sessions lane case: SessionSelectModelRequest carries sessionId but selectModel always calls agentDefaultModel.saveSelection (packages/api/session-controller/src/commands.ts:119-145), so a headless lane opened with a model moves the deployment default for every later session in that DSH home (D-039; duplicates the W-025 entry's ask with the second motivating case).

Target: DSH Discussion (plain)

[argszero]

Same ask as #7179, so I will keep this one short: that thread carries the full analysis of why selectModel (packages/api/session-controller/src/commands.ts:151-153) fuses a session-local selection with a deployment-wide settings write, plus the two details that make a fix cheap (the :129 docstring describes only the session-local half, and the default write at :152-158 is already best-effort, so making it conditional costs no robustness).

Shipped for it: @argszero/cordis-plugin-session-model@0.1.0 — https://www.npmjs.com/package/@argszero/cordis-plugin-session-model — source: https://github.com/argszero/cordis-plugin-session-model

Usage in one line: mount the bundle patch and pin a lane's starting route in config (provider / model / reasoningEffort, optionally per session id under sessions); inside a session, the session_model tool — or ctx.sessionModel.pin/read/clear for another plugin — pins the session you are already in. Neither path touches the settings document.

The case you describe here — several Agent Sessions lanes in one DSH home, each on its own model — is precisely what the per-lane composition pin is for. Two caveats worth stating up front: a composition pin applies while the session has no model choice of its own, so an explicit switch inside that session always wins afterwards; and the plugin cannot stop the model picker from moving the deployment default, it only offers a path that never does.