fix(pi): keep session command failures structural

[kvnloo]

Stacked on #13777. This resolves the remaining session-command error-surface review findings without changing Pi session behavior.

What changes

• ProviderSessionCommandError now carries the bounded command category (copy|export|share) plus the underlying cause instead of copying cause.message into the RPC-visible detail.
• RPC messages are structural: e.g. Failed to share provider session.
• failed gh gist create stderr stays out of PiRpcError.message; it is retained only as the underlying cause.
• tests assert provider/CLI text does not appear in the public error message.

Scope

No Pi turn/session lifecycle changes. No provider discovery or UI changes.

Validation

Focused regression coverage is included in:

• apps/server/src/provider/PiSessionCommands.test.ts
• apps/server/src/server.test.ts

This branch is an exact stack on #13777 head 5f344199fbd5ace71194302362dcb5b96a829d8b; upstream CI is the executable validation surface.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This small bug-fix changes the shared WebSocket error contract and forwards raw provider or GitHub CLI causes to clients. Those causes may contain sensitive paths, account details, or credentials, warranting human security review.

Not approved because:

• 1 blocking correctness issue found at or above your repo's Minimum Blocking Severity

^{Adjust the Minimum Blocking Severity for this repo — including turning it Off — in Settings. You can add or adjust custom eligibility rules. Learn more.}

[kvnloo]

Fixed the blocking review finding at 953b989c: the RPC error schema no longer contains cause, and the WebSocket mapping drops provider/CLI causes entirely. Raw gh stderr remains only inside the server-local PiRpcError cause; the serialized client error is structural (threadId + command) with a fixed message.

[juliusmarminge]

Note

This comment is posted by Julius' dot

The PR lists two regression suites but gives no executed results, and current-head CI is awaiting approval. The verification requirement requires observed outcomes for the changed behavior. Please run the focused error-serialization tests or link a completed CI run that includes them, then request reconsideration.