Stabilize agent orchestration and remove Plan/Goal support

[IchenDEV]

Outcome

Agent changes now carry bounded durable conversation context without nested handoff text, preserve session ownership during failures, and clean up adapter process groups. Provider/model choices come from installed integrations and live discovery; the UI shows switching and discovery failures explicitly.

Remove Plan/Goal controls and execution paths, including Plan-first scene/skill/mobile activation. Historical conversations, drafts and scenes remain readable without reactivating retired features.

Change record

• Change: docs/sdlc/changes/2026-09-28-orchestration-stability/intent.md
• Change: docs/sdlc/changes/2026-09-28-remove-plan-goal/intent.md

Validation

• Rust workspace check; affected Core and server regression suites, including HTTP/WebSocket mobile restart recovery.
• Desktop regression suites, lint, TypeScript, renderer build and actual browser rendering.
• Live stability canary covered all 20 directed pairs among five authenticated agents in one session with 64 Ki-character initial history. That matrix preceded Plan/Goal removal; post-removal validation used deterministic regressions. Account-blocked routes and other limitations are recorded in Verification.
• Documentation, worktree scope and diff checks passed locally. Remote CI must pass before merge.

Review and release

• Acceptance evidence covers the current diff, including rendered UI where affected.
• Required local checks passed; failures and skipped checks are explained in the records.
• Merge authorization: User explicitly requested "pr & merge" on 2026-09-28.
• Release authorization: Not requested.