v2.6.0 — multi-agent adapter migration

The multi-agent adapter migration (milestone v2.6.0). DevFlow stops rendering one shared
/gsd-* prompt for every agent and moves to a modular AgentDriver contract, where each agent
owns its prompt rendering, command building, completion parsing, and health. This fixes Codex
(which received GSD slash commands as literal shell strings) and registers Pi as a fourth,
selectable agent.

What's new

• Pi is a fourth, selectable agent (Phase 36). AgentKind::Pi + PiAgent with a
  pi auth check-backed health check that distinguishes "installed" from "headless-capable"
  (never env-var sniffing). Pi launches with --no-approve — --approve would trust
  project-local extensions that execute unsandboxed.
• The release path signs deterministically (999.104 / Phase 36). scripts/cut-release.sh
  fails loudly before tag -s when devflow.releaseSigningKey is unset or unreadable, instead of
  silently signing with whatever user.signingkey defaults to. The capability-only
  signing-viability probe is removed; the pre-push fingerprint hook — the only check that
  distinguishes the maintainer key from the agent key — is retained.
• release --check catches a forgotten version bump (999.96 / Phase 36). A new row compares
  CHANGELOG.md's top heading to the workspace version and reports NOT viable with direction.
• A modular AgentDriver contract replaces AgentAdapter (Phase 37). Each driver owns its
  prompt rendering, command building, completion parsing, health, and interactivity declaration,
  instead of that logic being scattered across prompt.rs, agents/*.rs, agent_result.rs, and
  preflight.rs.
• Codex is fixed (999.31 / Phase 37). The StageIntent de-Claude-ification removes the
  literal /gsd-* slash commands from every stage prompt; Codex now receives a workflow-file
  reference and the verified non-interactive approval flag (-a never, the global form).
• Pi is the second native driver (Phase 37). Pi runs on -p print mode with the same
  de-Claude-ified prompt; its end-to-end run (JSON unwrapper + monitor CloseRule) is a
  follow-on (37.1/38).
• A shared conformance suite (test_contract()) (Phase 37). Every driver must pass it; a
  deliberately-broken driver must fail it. InteractivityMode and DriverHealth are declared at
  the driver layer.

Deferred (recorded, not shipped here)

• AgentAdapter removal + InteractivityMode consumption (999.106).
• Codex parser success-before-failure + writable-root serialization (999.107, pre-existing).
• Pi end-to-end (JSON unwrapper + CloseRule) — 37.1/38.