feat(agents): agent platform — registry, activity hooks, harness allowlist

[yyovil]

Agent platform (base of a stacked series)

This is the root PR of a stack. It lands the shared platform that every
per-agent adapter plugs into; each subsequent PR in the stack adds one agent
adapter on top of this.

Wired here for the three already-shipped harnesses — claude-code, codex,
opencode.

What's in it

• adapters/agent/registry — single source of truth for the shipped
  adapters (Constructors()); the daemon resolves a session's harness through
  it. Adding an agent becomes one line here.
• adapters/agent/activitydispatch + the ao hooks command — maps an
  agent's native hook callbacks onto AO activity states
  (active/idle/waiting_input/…).
• claude-code / codex / opencode — emit SessionStart /
  UserPromptSubmit / Stop activity through the dispatcher.
• HTTP + OpenAPI — report session activity state.
• db — a single migration that widens sessions.harness to all shipped
  harnesses at once, replacing what was an 18-migration chain where each step
  textually depended on the previous one's output (and silently no-op'd if
  merged out of order). New adapters now need no further migration.
• domain — harness constants + an --agent alias for ao spawn.

Why a stack

The per-agent adapters are independent packages, but they all register through
the same handful of files (Constructors(), the activity dispatcher, the
harness allowlist). Landing the platform once, then stacking one adapter per PR,
keeps each adapter reviewable in isolation without a tangle of migration-order
and merge hazards.

Testing

go build / go vet / go test ./... green, except the pre-existing
TestSessionStreamsRealZellijPane integration test, which fails only on the
known environmental zellij IPC-socket-path-too-long issue (long macOS $TMPDIR)
— unrelated to this change.

🤖 Generated with Claude Code

[yyovil]

Superseded by #119 — recreating in-upstream so the adapter stack can base off it.