Remove global socket fallback, enforce local-only daemons

- Remove ~/.beads/bd.sock fallback in getSocketPath()
- Always return local socket path (.beads/bd.sock)
- Add migration warning if old global socket exists
- Update AGENTS.md to remove global daemon references
- Document breaking change in CHANGELOG.md
- Fix test isolation: tests now use temp .beads dir and chdir

Prevents cross-project daemon connections and database pollution.
Each project must use its own local daemon.

Amp-Thread-ID: https://ampcode.com/threads/T-c4454192-39c6-4c67-96a9-675cbfc4db92
Co-authored-by: Amp <amp@ampcode.com>

CHANGELOG.md

@@ -7,6 +7,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+- **BREAKING**: Removed global daemon socket fallback (bd-231)
+  - Each project now must use its own local daemon (.beads/bd.sock)
+  - Prevents cross-project daemon connections and database pollution
+  - Migration: Stop any global daemon and restart with `bd daemon` in each project
+  - Warning displayed if old global socket (~/.beads/bd.sock) is found
+
 ## [0.10.0] - 2025-10-20
 
 ### Added