perf(mcp): resolve list_projects branches with one Git child

Previous behavior: src/mcp/mcp.c:3895 called cbm_git_context_resolve for the single branch field. Attached repositories could execute seven Git commands per project, and a rejected status-based replacement took 3,380.410 ms for the isolated 50-project page.

Add src/git/git_context.c:115 resolve_current_branch as the single attached/detached/unborn implementation shared by cbm_git_context_resolve and cbm_git_current_branch. cbm_git_run_first_line_buf drains stdout and reaps symbolic-ref once; cbm_pclose_exit_code normalizes POSIX wait status and Windows GetExitCodeProcess results. Exit 0 returns attached/unborn names, exit 1 returns DETACHED, and non-Git errors remain absent.

Route CBM_ONLY_SUITE=git_context in tests/test_main.c:386 so focused runs execute four tests instead of traversing high-memory fixtures. tests/test_git_context.c:252 covers attached, detached, unborn, non-Git, full-context parity, heap cleanup, and portable command setup; tests/test_security.c:621 covers normalized exit 7.

Verification: Git context ASAN/UBSan 4/4; normalized-exit canary 1/1; MCP ASAN/UBSan 224/224; scripts/check-source-safety.sh passed; staged diff check passed. The -O2 binary returned the isolated default page in 773.848 ms and explicit full inventory in 5,023.220 ms, preserved 41/53 branch fields including the unborn main branch, and reaped both server processes and all reader threads.
Signed-off-by: Andrew Hundt <ATHundt@gmail.com>

src/git/git_context.h

@@ -21,6 +21,9 @@ typedef struct {
 
 int cbm_git_context_resolve(const char *path, cbm_git_context_t *out);
 void cbm_git_context_free(cbm_git_context_t *ctx);
+/* Resolve only the current branch for branch-only callers. Returns a heap
+ * string (including "DETACHED") through out_branch on success. */
+int cbm_git_current_branch(const char *path, char **out_branch);
 char *cbm_git_context_branch_qn(const char *project_name, const cbm_git_context_t *ctx);
 int cbm_git_context_props_json(const cbm_git_context_t *ctx, char *buf, int buf_size);