#7868: Default DM sessions to per-channel scope (avoid webchat contention)

## Summary - Default DM session scope to per-channel-peer for non-webchat channels. - Keep webchat on main to preserve UI continuity. - Add tests for the new default behavior. ## Why Feishu and other non-webchat DM channels currently collapse into the main session when `session.dmScope` is unset. This allows webchat to contend for the same session and can mis-route replies. Using per-channel-peer by default isolates channels while keeping webchat unchanged. ## Tests - `resolve-route` unit tests updated. (Full test suite requires pnpm.) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR changes the default DM session scoping used by routing/session-key generation: when `cfg.session.dmScope` is unset, non-`webchat` DM channels now default to `per-channel-peer` (isolating each channel+peer into its own session key), while `webchat` keeps the historical default of collapsing DMs into the agent’s `main` session key for UI continuity. Unit tests in `src/routing/resolve-route.test.ts` were updated/added to lock in the new defaults. <h3>Confidence Score: 5/5</h3> - This PR appears safe to merge and matches the stated routing/session scoping intent. - Changes are small, localized, and covered by updated/added unit tests. The new default dmScope selection only affects DM peers (non-DM session keys are unchanged), and `webchat` explicitly preserves the previous main-session behavior. - No files require special attention <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->