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.

internal/storage/sqlite/schema.go

@@ -129,12 +129,6 @@ CREATE TABLE IF NOT EXISTS export_hashes (
     FOREIGN KEY (issue_id) REFERENCES issues(id) ON DELETE CASCADE
 );
 
--- Issue counters table (for atomic ID generation)
-CREATE TABLE IF NOT EXISTS issue_counters (
-    prefix TEXT PRIMARY KEY,
-    last_id INTEGER NOT NULL DEFAULT 0
-);
-
 -- Child counters table (for hierarchical ID generation)
 -- Tracks sequential child numbers per parent issue
 CREATE TABLE IF NOT EXISTS child_counters (