feat(home-registry): Home Registry + dashboard Home picker

[ThomasK33]

Summary

Adds a Home Registry + dashboard Home picker so agent-tty can see across the many Homes (state roots) you run it in — the default ~/.agent-tty, throwaway mktemp -d dirs, and per-project AGENT_TTY_HOME overrides — instead of only the single Home resolved for one invocation.

Closes #130. Design captured in CONTEXT.md (Home / Active Home / Home Registry terms) and docs/adr/0008-home-registry-advisory-per-machine.md.

What's new

• Home Registry — a persisted, advisory, per-machine index at ${XDG_STATE_HOME:-~/.local/state}/agent-tty/homes.json ({path, lastSeenAt} entries; counts derived live, never cached). Auto-registered the first time a Home hosts a Session (create); the write is advisory and never fails creation. Atomic temp+rename tolerates concurrent creates.
• home list [--all] [--json] — registered Homes (Active Homes by default; --all adds terminal-only), each row showing path, live active/total Session counts, and last-seen, newest-first. home forget <path> deregisters a Home without touching disk.
• Dashboard Home picker — press H for a full-screen picker; select a Home to observe its Sessions and open a Live View. Browsing Homes is a read-only scan that never reconciles; entering a Home reconciles exactly as the single-Home dashboard does today. Opens on the launched/resolved Home (additive — nothing about today's behavior changes).
• gc is now cross-Home by default — sweeps every registered Home and deregisters emptied/deleted ones; --home/AGENT_TTY_HOME scopes to a single Home and touches no registry. gc never deletes a Home directory (only Session directories + registry entries).

⚠️ Backward-incompatible

Plain agent-tty gc changes from "collect the default Home" to "sweep all registered Homes", and its --json result changes shape to { dryRun, homes[], removedSessionCount, totalBytesFreed, deregisteredHomes } (per-Home outcomes live under homes[]). Automation relying on the old behavior or the old flat shape must pass --home and read homes[]. Called out in CHANGELOG.md.

Boundaries (ADR 0008)

Per-machine and local-only: reconcile/kill judge liveness with local PIDs and manifests carry no machine identity, so a Home shared across machines (e.g. NFS) is unsupported; a machine-id guard is deferred. This matches the Coder model (separate workspaces are separate machines, each with its own registry).

Testing

• New unit tests: registry store (dedupe, prune-on-read, atomic write, XDG resolution, independence from AGENT_TTY_HOME), the read-only scope scan (including a guard that a dead-host running manifest is not reconciled by listing), home list/home forget envelopes, cross-Home runGcCommand (scoped vs sweep, deregister-on-empty), create auto-registration + advisory-failure tolerance, and golden-envelope contracts for the new/changed result shapes.
• New host-free integration test for the cross-Home gc sweep + deregistration (never deletes the Home dir).
• typecheck, oxlint, oxfmt --check, the full unit suite, and build pass locally. (Host-dependent integration/e2e run in CI.)

Notes for reviewers

• ADR numbering: 0008 currently collides with the unpushed feat/workflow-hero-demo branch's 0008-carry-two-parallel-hero-demos — whichever merges second must renumber.

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