feat(team-inbox): land notification and collaboration followups

[beruro]

Problem

PR #582 was merged into the already-landed #566 head branch, and PR #583 was then merged into the #582 head branch. Both PRs show as merged, but their notification and Team Inbox changes never reached develop.

Solution

• replay the exact feat(notifications): unify background delivery lifecycle #582 notification commit and feat(team-inbox): harden collaboration handoffs #583 Team Inbox commit, in order, on the latest develop
• preserve the original two-commit boundary and exclude all already-landed perf(cli): replace status polling with turn-intent push #565/fix(session): preserve queue and terminal finality #566 changes
• verify the recovery diff matches the original combined patch id exactly
• land background notification lifecycle, atomic collaboration handoffs, stable mention identity, and bounded realtime signal coalescing

Potential risks

• the recovery touches 98 files across TypeScript and Rust, so merge conflicts are possible if develop changes the same Team Inbox or notification paths before landing
• notification delivery and realtime teardown are lifecycle-sensitive; regressions could cause duplicate delivery or retained timers
• mitigations: exact patch-id match, clean replay on current develop, 88 targeted tests, TypeScript typecheck, and cargo check -p org2 --lib all pass
• do not merge the old feat(notifications): unify background delivery lifecycle #582/feat(team-inbox): harden collaboration handoffs #583 branches into develop again after this PR lands