feat: add opt-in post-task experience drafts

[Sycun]

Useful know-how produced during a run currently dies with the transcript. This adds an opt-in, draft-only learner: when a task finishes, a bounded background job distills the run's tool evidence into experience drafts. Nothing is promoted automatically — there is no activation endpoint, drafts are never installed into the skills directory, and no skill, role or tool is enabled by this change. A human reads them in the console.

Boundaries:

• learning.enabled defaults to false, is captured at startup and requires a restart, matching the documented hot-apply boundaries.
• One worker goroutine polling every 5s, <=20 jobs per UTC day (cap 200), 60s model timeout (cap 120), 12000 runes of evidence per job (cap 32000, <=2000 per item), <=3 attempts before a job is dropped.
• A draft must cite at least one tool_result; fragments marked truncated may not be reasoned about. Drafts containing protected configuration values are rejected before storage, and the candidate manifest goes through skillpackage.ValidateSkillMDPackage.
• Storage: one learning_jobs table plus two queue/conversation indexes. API: one protected read-only route, GET /api/conversations/:id/learning. UI: a chat toolbar entry gated on chat:read, with 11 keys in each of zh-CN / en-US / ru-RU.
• AgentTaskManager gains SetRunPersistenceHooks so the learner reads from the existing finalization path instead of adding a second write site; the hooks only do bounded persistence and never call a model.

Validation:

• go build ./...
• go test ./... across the repository, no failures
• go test -count=1 ./internal/learning/... ./internal/config/... ./internal/database/... ./internal/handler/...
• node --test web/static/js/*.test.cjs -> 320/320 pass
• Rebased onto main after fix: serialize SQLite asset writes without multiplying busy waits #362; the web/templates/index.html change is purely additive and leaves existing ?v= asset versions untouched.

Deliberately excluded: no standalone guide or research note, since 63955c4 just removed internal process journals from docs/. The single documentation edit is a ## 任务经验草稿 section in docs/zh-CN/configuration.md describing the new keys, written to match the existing project.context_checkpoints entry.

[Sycun]

Keeping this experimental learner local for now; not opening an upstream PR. Will re-submit if it earns one.