feat(story,scheduler): add epic-proposal tool + AskUser-timeout escalation (Phase 7c)

Two independent pieces, completing Phase 7.

Epic-proposal tool: AgentChannel gains a 5th method, ProposeEpic(ctx,
EpicProposal{Name, Description, StoryIDs}) (epicID, err), implemented on
storeChannel -- matches an existing epic by exact name or creates one
(DiscoverySource: "agent"), sets epic_id on each resolvable story (skips,
doesn't fail, on an unresolved ID), emits KindEpicProposed attached to the
epic's own ID with payload {epic_id, name, story_ids}. Wired into both
transports exactly like Phase 6 wired role into spawn_subtask: a new
propose_epic tool in the native tool-use loop (internal/agentloop/tools.go)
and the MCP transport (internal/executor/agentmcp.go). This is the mechanism
for a discovery/planner-role agent to act on its own judgment that several
stories it's been given form one cohesive initiative -- the judgment itself
lives in the calling agent's instructions/model, not in this code.

AskUser-timeout escalation: extends the existing Scheduler (Phase 5's
retry-then-escalate watcher) rather than adding a new component, since
"stuck task needs escalation" is exactly what it already does. Finds
role-typed BLOCKED tasks whose question has been outstanding longer than
SchedulerConfig.AskUserTimeoutSeconds (default 10 minutes) using
task.UpdatedAt as the outstanding-since timestamp -- no new column needed,
since UpdateTaskQuestion already stamps it the instant a question is
recorded and nothing else touches the row while BLOCKED. Resolves the next
ladder tier from the latest execution's EscalationRung, records the
system-authored fallback answer as an audit-trail task.Interaction, clears
the question, sets the new tasks.needs_review flag, emits KindEscalated
(now carrying a trigger field: "failure" vs "ask_user_timeout" for the
existing failure-retry path vs this one), and resumes via Pool.SubmitResume
at the escalated tier -- degrading to same-tier resume with final:true if
the ladder's exhausted or no role config exists, since unblocking the task
takes priority over having somewhere higher to escalate to.
GET /api/tasks?needs_review=true surfaces auto-decided tasks for human
review.

go build/vet/test -race -count=1 all pass, full suite (20 packages), run
twice to rule out flakiness in the new tests. (One pre-existing, unrelated
test -- TestHandleRunTask_CascadesRetryToFailedDeps, a tempdir-cleanup race
-- appeared once under full-suite load per the implementing agent's report
and did not reproduce in this verification's runs either; not a regression
from this work.)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V1moSNCJRcP6kykA4tyUSs

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