ChangeDir does not affect automatic Copilot session selection or MCP path policy

Peer-review finding

Reviewed snapshot: 5c38677469756c69f86678c0b907d82e15d0a539.
The reviewer and parent independently reproduced this against unchanged source.
Only fresh synthetic session homes and a fixed non-mutating native stub were
used; no real conversation history, user configuration or Copilot process was
accessed. Do not run destructive reproduction commands against real sessions.

Severity: Medium. Confidence: 10/10.
Suggested existing labels: bug, module:copilot.

Locations: modules\Shmuelie.Copilot\Get-CopilotLaunchPlan.ps1:599-610,677,731;
modules\Shmuelie.Copilot\SessionSelection.ps1:16-49.
Start-Copilot forwards the parameter without changing location at
modules\Shmuelie.Copilot\Start-Copilot.ps1:582-602.

The parameter's help promises to change directory before other work, but the
implementation only appends -C to the future CLI arguments. The launcher itself
continues evaluating MCP path globs and resume candidates against Get-Location
in the original directory.

Minimal synthetic reproduction:

• Create actual scratch directories A and B, and one fake session per directory.
• Configure fake-home MCP server only-a with autoConnect = @($directoryA) and
  server only-b with autoConnect = @($directoryB).
• Stub native Copilot command discovery and Git branch lookup; do not launch
  Copilot.
• From A, inspect Get-CopilotLaunchPlan -ChangeDir $directoryB, then
  Start-Copilot -PassThru -ChangeDir $directoryB.

Actual: Both plans select A's session. The plan forwards -C B but disables
only-b and leaves only-a enabled.

Expected: B's session is selected and only-a is disabled, with policy evaluated
for B. The caller's original location remains unchanged after planning/launching.

Acceptance criteria: Resolve the effective launch directory before all
directory-sensitive planning, consistently across both entry points. Cover absolute
and relative ChangeDir, session/branch selection, MCP globs, and caller-location
restoration on success and error. Preserve explicit resume and selector semantics.
No assumption about an unverified native CLI flag is needed: the wrong plan is
observable without launching.

Read modules\Shmuelie.Copilot\Get-CopilotLaunchPlan.ps1 and SessionSelection.ps1, then compare how Start-Copilot.ps1 forwards ChangeDir. Run the synthetic A/B reproduction with native Copilot discovery and Git branch lookup stubbed, without launching Copilot. Done means both entry points plan against the effective directory for absolute and relative paths, preserve explicit resume and selector behavior, and restore the caller location on success and error.