Add backend management specs and @spec annotations (#727)

* Add backend management specs and @spec annotations

- Curate specs/backend-management.md with 4 behavioral specs (BM-001–BM-004)
- Add @spec comments to source and test files for traceability
- Add spec file convention note to AGENTS.md

Co-authored-by: openhands <openhands@all-hands.dev>

* Remove duplicate BM-002 test (non-conversation redirect)

The 'stays on settings' case was covered by two tests; keep the one
with the clearer assertion and drop the redundant copy.

Co-authored-by: openhands <openhands@all-hands.dev>

* Parameterize BM-002 tests with it.each

Collapse 3 identical-structure redirect tests into a single
parameterized it.each covering conversation detail, automation detail,
and non-ID routes.

Co-authored-by: openhands <openhands@all-hands.dev>

* Document @spec tagging convention in AGENTS.md

Co-authored-by: openhands <openhands@all-hands.dev>

---------

Co-authored-by: openhands <openhands@all-hands.dev>

specs/backend-management.md

@@ -1,4 +1,12 @@
 # Backend Management Specs
 
-## BM-001: Auto-switch on connect
+---
+
+### BM-001: Auto-switch on connect
 - [x] Adding a backend shall automatically switch the active selection to it.
+
+### BM-002: Switching backends keeps the user on the same page
+- [x] Switching backends shall redirect to the same section but on the new backend. The user shall never see stale data from the previous backend.
+
+### BM-003: Fallback on active backend removal
+- [x] Removing the currently active backend shall fall back to a remaining local backend. The user shall never be left without an active backend.