feat: plan-first default agent for all Amicode sessions

[aarontrowbridge]

What. Every new Amicode session opens on opencode's built-in read-only plan agent, not straight into the build/execute agent. The user switches in the composer to pulse-designer or build when they're ready to operate.

How. buildOpencodeConfigContent in packages/extension/src/opencode_config.ts now injects default_agent: "plan" into the config blob every Amicode server boots with. OPENCODE_CONFIG_CONTENT deep-merges over the user's global config, so this is the product default for all users regardless of per-machine settings. Per-message agent (the e2e tests, the distiller's --agent) is unaffected.

Verification. Fast suite green: 867 passed / 3 skipped / 0 failed. The vendored opencode 1.17.3 binary accepts default_agent: "plan" via the real-binary injection+merge test.

[Rchari1]

Completed in 053f952 Modes: default to plan, retire the pulse-designer agent shell (#389) (#395) — buildOpencodeConfigContent at packages/extension/src/opencode_config.ts:434 now injects default_agent: "plan" (deep-merges over user global config; per-message agent unaffected). PR #301 superseded and closed. Fast suite 867/3/0 + vendored 1.17.3 injection test at opencode_config.test.ts:96-99, 203-251.