Implement 6-char progressive hash IDs (bd-166, bd-167)

- Hash ID generation now returns full 64-char SHA256
- Progressive collision handling: 6→7→8 chars on INSERT failure
- Added child_counters table for hierarchical IDs
- Updated all docs to reflect 6-char design
- Collision math: 97% of 1K issues stay at 6 chars

Next: Implement progressive retry logic in CreateIssue (bd-168)
Amp-Thread-ID: https://ampcode.com/threads/T-9931c1b7-c989-47a1-8e6a-a04469bd937d
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/schema.go

@@ -135,6 +135,14 @@ CREATE TABLE IF NOT EXISTS issue_counters (
     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 (
+    parent_id TEXT PRIMARY KEY,
+    last_child INTEGER NOT NULL DEFAULT 0,
+    FOREIGN KEY (parent_id) REFERENCES issues(id) ON DELETE CASCADE
+);
+
 -- Issue snapshots table (for compaction)
 CREATE TABLE IF NOT EXISTS issue_snapshots (
     id INTEGER PRIMARY KEY AUTOINCREMENT,