#22098: fix: isolate agent sessions by explicit --session-id

## Summary - In `resolveSessionKeyForRequest`, when `--session-id` does not match an existing entry, we now create a dedicated session key instead of falling back to the default agent key. - Existing behavior is preserved for explicit `--session-key` and reverse lookup cases (`--session-id` matches a saved session in any configured agent store). - Added a deterministic dedicated key shape: `agent:<agent-id>:session:<normalized-session-id>`. - Added unit coverage for fallback creation and `--agent`-scoped session-id isolation. ## Testing - `pnpm vitest run src/commands/agent/session.test.ts` Closes #22085 <!-- greptile_comment --> <h3>Greptile Summary</h3> This PR fixes session isolation for explicit `--session-id` flags by synthesizing a deterministic session key (`agent:<agentId>:session:<sessionId>`) when a provided session ID doesn't match any existing entry across all agent stores. Previously, this scenario returned `undefined` for the session key, which could cause unintended session reuse or failures. - Adds `normalizeSessionIdSessionSuffix` and `buildSessionIdSessionKey` helpers to sanitize and construct isolated session keys - Splits the former `explicitSessionKey` variable into `explicitSessionKey` (user-provided `--session-key`) and `explicitAgentSessionKey` (resolved from agent config), enabling the new fallback to distinguish between the two - New fallback block fires only after primary store lookup and cross-store search both fail to find the session ID - Tests updated to reflect the new behavior and cover the `--agent` + unknown session ID case <h3>Confidence Score: 4/5</h3> - This PR is safe to merge — the new fallback is well-gated behind existing resolution paths and only fires as a last resort for unknown session IDs. - The change is narrowly scoped to a single function with clear before/after behavior. The new session key namespace (`session:`) doesn't collide with existing patterns. Tests cover both the default agent and explicit agent scenarios. The only concern is a minor naming style issue. - No files require special attention. _{Last reviewed commit: 4d12ebd} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->