feat(ai): agent run persistence (AI-036)

[mrviduus]

Phase 6 AI-036 — agent runs are saved so the reader UI can replay an agent's steps (AI-038) and runs are observable. Persistence mechanics only; the endpoint that calls the writer is AI-037.

Changes

• agent_run table (AddAgentRun migration) + AgentRun entity + DbSet on IAppDbContext/AppDbContext (AppDbContext.Agents.cs). Columns: agent / user / edition / goal / status / output / steps_json (jsonb) / iterations / tokens / cost / latency / error / created_at. Optional FK→users ON DELETE SET NULL (a deleted user doesn't erase history); partial index on (user_id, created_at).
• IAgentRunWriter (Ai.Core) + DbAgentRunWriter (Application/Ai, scoped — mirrors DbLlmTraceWriter): flattens the framework-free AgentRunRecord into the entity, serializing the step transcript to jsonb. Awaited by the caller (the run is already finished — no latency to hide, recommendation Add Claude Code GitHub Workflow #3).
• AgentRunRecord (Ai.Core) + AgentRunRecordFactory (Ai.Agents): Completed / BudgetExhausted / Failed build a persistable record uniformly across outcomes.
• Budget-exhausted runs keep their transcript (recommendation Fix build errors: Add missing packages and NuGet source configuration #1): AgentBudgetExhaustedException now carries the partial Steps + Usage accumulated before the cap — the run you most want to inspect is the one that ran out of budget. AgentLoop throws with them at both the cost-cap and max-steps exits (additive; AI-034 tests still pass).
• steps_json is a single jsonb column (recommendation feat: add full-text search library with PostgreSQL FTS #2) — read whole for "show steps", not queried per field.

Verification

• AgentRunRecordFactory tests (completed / budget-exhausted-keeps-transcript / failed); AgentLoop budget-exhaustion test now asserts the exception carries the partial transcript + usage.
• Migration applied locally; \d agent_run + a jsonb insert/read roundtrip + FK + partial index verified on Postgres.
• Full TextStack.UnitTests (267) + TextStack.AiEvals (28) green; dotnet format clean.

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