Modes: default to plan, retire the pulse-designer agent shell (#389)

[aarontrowbridge]

Closes #389

Two commits:

1. Plan-first default (cherry-picked from Aaron's 2026-08-08 feat/plan-first-default work, refs feat: plan-first default agent for all Amicode sessions #300, never merged): buildOpencodeConfigContent injects default_agent: "plan" — every new session opens read-only; execution starts on an explicit switch to build. Verified against the real vendored binary via the injection+merge test.

2. Retire the pulse-designer agent shell: the picker is plan/build only. The agent entry was a four-line prompt deferring to the compiled AGENTS.md interview section; its permissions were always the config-root block, so it did nothing build didn't. The interview content stays, agent-agnostic, in the injected instructions — the roles-not-modes posture from Autoresearch Studio — the loop is the product (pulse-designer → domain pack, telaio spine, campaign autonomy) #368. e2e turns send agent: "build"; GET /agent now asserts the retirement.

Verification: 920/920 fast suite, typecheck clean, real-binary config injection+merge green. The live-creds e2e (interview tier C) times out on a 300s model POST — reproduced identically on clean main in a throwaway worktree, so it's an environmental live-endpoint hang (overnight), not a regression; flagged separately.

Note: auto-merge is NOT armed — #389 is hitl.

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📥 Commits

Reviewing files that changed from the base of the PR and between 84a0784 and da19513.

📒 Files selected for processing (6)

• AGENTS.md
• packages/extension/src/opencode_config.ts
• packages/extension/src/scores/compiler.ts
• packages/extension/test/opencode_config.test.ts
• packages/extension/test/slow/interview_e2e.test.ts
• packages/extension/test/slow/scores_e2e.test.ts

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