Add Trail Runner Studio's demonstration-to-trail flow, parameterized trails, and workspace-aware default targets

[handstandsam]

Summary

Syncs the latest changes through 2026.07.14, bringing in:

• Trail Runner Studio: a "Create by demonstration" flow (record a human demonstration on-device, then an agent authors the durable trail) plus the ability to run Claude or Codex conversations inside Trail Runner itself for authoring, with a bundled skills/trailblaze-author skill for the agent side.
• Parameterized trails: a new arg-binder system (TrailArgBinder, TrailArgConfig, TrailArgTokens) lets a trail declare typed arguments and have callers bind values at dispatch time, with AgentMemory extended to seed initial memory from those args.
• Workspace-aware target config: TrailblazeWorkspaceConfigResolver resolves the default app target from a committed workspace defaults.target file instead of a hardcoded per-distribution default.
• MCP subprocess reliability: a bounded handshake watchdog force-kills a script: MCP tool subprocess that hangs during startup, instead of wedging the whole daemon.
• Trail migration fidelity: a new TrailRoundTripFidelityVerifier checks that migrating a trail to the unified YAML format preserves its recorded tool calls.
• Configurable trailheads in the Studio session brief, a fix for on-device runs typing the literal {{var}} instead of the remembered value, and CLI improvements across check, run, migrate-trails, and waypoint migrate-trail.
• 6 clock example trails migrated from the split-folder format to the unified single-file .trail.yaml format.

Implementation detail

Main touch points: trailblaze-host/.../trailrunner/ExternalAgent*.kt + BundleStore.kt (Studio's agent/demonstration backend), the Trail Runner web app's create.tsx/demonstrate.tsx/external-agents.tsx screens, trailblaze-models/.../yaml/TrailArg*, trailblaze-common/.../config/project/TrailblazeWorkspaceConfigResolver.kt, trailblaze-scripting-subprocess/.../McpSubprocessSession.kt, and trailblaze-host/.../migration/TrailRoundTripFidelityVerifier.kt.

Test plan

• CI passes on this PR (build, tests, lint)