fix(database): unify session DB — remove channel-based path splitting

[jeonghun-jj-lee]

Closes #188

Summary

Removes all channel-based database path logic. All channels now write to opencode.db. A one-time consolidation on first startup merges rows from legacy channel-named DBs (opencode-*.db) via ATTACH + INSERT OR IGNORE per table, then backs up sources as *.db.merged.

Changes

• Removed: STABLE_CHANNELS, consolidateLocalDb, InstallationChannel import, and OPENCODE_DISABLE_CHANNEL_DB env var logic
• Simplified path(): returns join(Global.Path.data, "opencode.db") for all channels (OPENCODE_DB env var override retained)
• Added consolidateChannelDbs(targetPath): runs once synchronously at startup via bun:sqlite:
  • Finds all opencode-*.db files (skips *.db.merged)
  • If opencode.db doesn't exist, renames the largest source to become it (no full copy)
  • Merges remaining sources: checkpoint WAL, ATTACH, INSERT OR IGNORE per table, DETACH, rename to *.db.merged
  • Foreign keys disabled during merge, re-enabled after
  • Idempotent: partial failures leave sources un-renamed for retry on next startup
  • Skipped entirely when OPENCODE_DB env var is set

Standalone script

A merge-opencode-dbs.sh bash script (using sqlite3 CLI) is available separately for users who want to merge before upgrading.

Summary by CodeRabbit

• Data Management
  • Consolidated channel-specific databases into a single opencode.db.
  • Automatically merges existing databases during startup while preserving records and removing duplicates.
  • Retains support for explicitly configured database paths.
  • Improves recovery with retry handling, backups, and cleanup of temporary database files.
• Documentation
  • Added documentation describing the unified database approach and migration behavior.

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 151d263 and f1aa7d8.

📒 Files selected for processing (2)

• docs/adr/0004-unified-session-database.md
• packages/core/src/database/database.ts

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📝 Walkthrough

Walkthrough

The PR unifies session storage in opencode.db. Startup consolidates legacy channel databases with deduplication, backups, WAL cleanup, retries, and largest-file promotion. Explicit OPENCODE_DB overrides remain supported. The ADR documents the behavior and design decisions.

Changes

Unified session database

Layer / File(s)	Summary
Legacy database consolidation packages/core/src/database/database.ts	Startup discovers opencode-*.db files, merges tables with INSERT OR IGNORE, checkpoints WAL files, handles foreign keys and transactions, renames merged sources, removes sidecars, and retries failures.
Canonical database routing packages/core/src/database/database.ts, docs/adr/0004-unified-session-database.md	Default routing uses opencode.db and removes channel-specific path logic. The ADR documents consolidation, backup, override, and migration behavior.

Estimated code review effort: 4 (Complex) | ~45 minutes

Suggested reviewers: jlongster

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch unified-session-db

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