a-change-runs-in-its-own-worktree: a change in its own worktree runs there

[VeryComplexAndLongName]

Summary

The owner's run of a HppMCP change from its card (2026-10-10) failed at its first stage:

cwd "C:/Prog/.worktrees/HppMCP/mcp-platform-foundation" is outside the workspace "c:\Prog\HppMCP"

Every agent was bound to the open workspace, and the chain, a single stage and a card's run controls chose an agent by its id alone — so every run in a change's own worktree was refused, in both hosts (openspec/changes/a-change-runs-in-its-own-worktree).

• core: runRootOf / agentsByRunRoot — a run inside one of the repository's worktrees (<worktree root>/<repository>/<dir>) gets that worktree's own agents, made once; any other run the workspace's, whose sandbox still refuses it.
• HarnessChainDeps.resolveRunner(agentId, cwd?); the editor's AI panel, chain and card run controls, and the standalone server's chain and socket runs pass the run's cwd.
• The editor's run dialog reads and writes the change's harness where the change is worked (before: the checkout's copy).

Test plan

• typecheck, lint (0 errors)
• core run-root 3 + chain runner; extension unit tests; server 110
• openspec validate --strict; merge gate names only 4.3 open
• e2e in CI
• 4.3 Human-only: Run Change... on a worktree card in HppMCP runs its stages

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