fix(version): remove git subprocess from bd version command (GH#1zonaz)

Under high concurrency (17+ agent sessions), each gt command invokes
bd version to validate minimum beads version. The resolveBranch()
function was spawning git subprocesses (git symbolic-ref) which caused
severe contention and timeouts when many agents ran simultaneously.

Changes:
- Remove git subprocess fallback in resolveBranch()
- Branch info now only comes from ldflags or build info's vcs.branch
- Version number (what gt actually checks) is unaffected
- Update documentation to reflect the change

The fix reduces bd version from potentially 2 git subprocesses to zero,
eliminating the contention issue under high agent concurrency.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/docs.md

@@ -69,6 +69,7 @@ The CLI is built on the Cobra framework and consists of command implementations
 
 **Fallback Resolution Chain** (important for development):
 - The `resolveCommitHash()` function first checks the ldflag, then checks runtime build info, returning empty if neither is available
+- The `resolveBranch()` function checks ldflags, then build info's vcs.branch - it does NOT spawn git subprocesses (GH#1zonaz: prevents contention under high concurrency)
 - This allows the version command to work even in development environments where ldflags aren't set (useful for testing)
 
 **Testing Coverage** (`@/cmd/bd/version_test.go`):
@@ -85,6 +86,6 @@ The CLI is built on the Cobra framework and consists of command implementations
 **Platform-Specific Considerations**:
 - The git extraction in Makefile uses POSIX shell constructs compatible with bash on all platforms
 - Windows builds via goreleaser set ldflags identically to Unix platforms
-- The `symbolic-ref` fallback in resolveBranch works reliably even in fresh repos with no commits
+- Branch info comes from build-time ldflags or vcs.branch in build info - no runtime git calls (GH#1zonaz)
 
 Created and maintained by Nori.