feat(status): collapse session status to five glanceable states (#332)

[harshitsinghbhandari]

Implements the resolved design in #332: the dashboard's job is to let you scan a wall of agents and tell at a glance which need you. Today one raw signal expands into 13 SessionStatus values run through three independent frontend groupings, most collapsing to the same move, so they add states to scan without changing the decision. This derives the states from the distinct moves instead.

The five states

State	Means	Your move
Working	the agent is actively running	leave it alone
Needs input	the agent is blocked on you	respond
Ready	a clean PR waits on you (mergeable / approved / review-required)	merge it / go review it
Stalled	the agent will not finish on its own (hung, never booted, or stopped with unfinished work)	get it moving
Idle	nothing happening, or finished (incl. merged / terminated)	nothing

What changed

Backend (single source of truth):

• service/session/status.go — deriveStatus now reads activity.state + raw PR buckets (clean / unfinished / neutral) directly, never a pre-collapsed status, so active-vs-PR precedence resolves the right way. First-match-wins ordering: terminated → needs_input → stalled → working → ready → idle.
• New stalled() detector: never-booted (boot grace), hung mid-run (hangTimeout), and stopped-with-unfinished-work. noSignalGrace → bootGrace, bumped 90s → 120s. hangTimeout is the one open number (TBD per the spec); set to 10m as a false-positive-averse starting point.
• Blocked stacked children keep their problem signals (unfinished) but have their readiness suppressed.
• domain/status.go — vocabulary reduced from 13 to 5. Dropped the worst-wins severity ladder, aggregatePRStatus, prPipelineStatus, statusSeverity, anyMerged.

Frontend (render verbatim):

• Deleted workerDisplayStatus, attentionZone, sessionNeedsAttention, workerStatusPulses, and their types/tables.
• Added one shared STATUS_META (+statusOrder) that the pill (topbar + inspector), board columns, sidebar dot, and card badge all read, so the surfaces can no longer disagree.
• Liveness (sessionIsActive, done-bar archive, restore, orchestrator liveness) now keys off a carried isTerminated fact rather than the removed merged/terminated statuses. The wire already carries isTerminated; no API/openapi enum change needed (status is a plain string).
• Board is now 5 columns (Needs input · Stalled · Ready · Working · Idle); terminated sessions archive in the existing done bar.

Net LOC

-237 LOC (346 insertions, 583 deletions), excluding the regenerated routeTree.gen.ts and lockfiles. Net negative as required.

Verification

• Backend: go build ./... clean; go test -race ./... green except two pre-existing internal/adapters/runtime/zellij tests (TestRuntimeIntegrationUsesExactSessionParsing, TestAttachCommandDisablesPaneFrames) that are specific to the local zellij 0.44.3 binary and untouched by this change.
• Frontend: tsc --noEmit clean; vitest run 131 passed.

Closes #332.

[harshitsinghbhandari]

Thanks for the thorough review. No blocking issues; addressed the actionable nits in 728c8dd:

• Em dashes (hard repo style rule): fixed the activityDetail "Stalled" copy and the doc comments I authored in workspace.ts. Left the pre-existing PRCard "—" empty-value placeholders and the timeline separator alone, since they predate this PR and aren't prose I produced.
• Stale no_signal prose (nit chore: scaffold backend/ and frontend/ skeletons for rewrite #1): swept the six comments in service.go, lifecycle/manager.go, daemon/lifecycle_wiring.go, and activitydispatch/dispatch.go to the new "never-booted / broken-pipeline stall" vocabulary. The signalCapable plumbing they describe is unchanged. (Left session_store.go's "zero = no signal received yet" as-is; it describes the zero-time convention, not the removed status name.)
• Degenerate-stack fallback (behavior feat(backend): Lifecycle Manager + Session Manager lane #2): added a comment in prMoves making the cyclic-all-clean → Idle behavior an explicit, conscious choice. Not adding a guard: it requires a cycle of mergeable PRs each targeting the next's branch with no root, which can't arise from a real branch topology, and the old force-aggregate was defensive code for a case that can't occur.

Conscious sign-offs on the non-blocking behavior notes:

• chore: scaffold backend/ and frontend/ skeletons for rewrite #1 (merged-but-not-terminated no longer auto-archives): intentional per Session status model — final structure (5 states: Working / Needs input / Ready / Stalled / Idle) #332 (liveness moved to the carried isTerminated fact). Such a session shows in the Idle column until the reaper tears it down, rather than the done bar. Accepting this.
• Tracking: Lifecycle Manager + Session Manager lane #3 (hangTimeout = 10m): kept as the spec's one explicitly-open number; documented in code as tunable. Will revisit against live sessions.
• feat(decide): pure DECIDE core + exhaustive truth-table tests #4 (active-wins blind spot): carried forward unchanged, documented in spec and code.
• STATUS_META.attention: wired into the shared table and tested; the sidebar emphasis/counter surface that would consume it isn't built yet (same as the old test-only sessionNeedsAttention). Reserved for when that surface lands.

Backend go build + touched-package go test -race green; frontend tsc + vitest (131) green.