Remove sequential ID code path (bd-aa744b)

- Removed nextSequentialID() and getIDMode() functions
- Removed issue_counters table from schema
- Made SyncAllCounters() a no-op for backward compatibility
- Simplified ID generation to hash-only (adaptive length)
- Removed id_mode config setting
- Removed sequential ID tests and migration code
- Updated CONFIG.md and AGENTS.md to remove sequential ID references

Follow-up bd-2a70 will remove obsolete test files and renumber command.

beads_twoclone_test.go

@@ -48,7 +48,6 @@ func TestTwoCloneCollision(t *testing.T) {
 	t.Log("Initializing beads in clone A")
 	runCmd(t, cloneA, "./bd", "init", "--quiet", "--prefix", "test")
 	// Enable hash ID mode for collision-free IDs
-	runCmdWithEnv(t, cloneA, map[string]string{"BEADS_NO_DAEMON": "1"}, "./bd", "config", "set", "id_mode", "hash")
 	// Configure git to use merge instead of rebase (sorted JSONL merges cleanly)
 	runCmd(t, cloneA, "git", "config", "pull.rebase", "false")
 	
@@ -65,7 +64,6 @@ func TestTwoCloneCollision(t *testing.T) {
 	t.Log("Initializing database in clone B")
 	runCmd(t, cloneB, "./bd", "init", "--quiet", "--prefix", "test")
 	// Enable hash ID mode (same as clone A)
-	runCmdWithEnv(t, cloneB, map[string]string{"BEADS_NO_DAEMON": "1"}, "./bd", "config", "set", "id_mode", "hash")
 	// Configure git to use merge instead of rebase (sorted JSONL merges cleanly)
 	runCmd(t, cloneB, "git", "config", "pull.rebase", "false")