Add redo-reset command

[dinkelk]

Summary

Adds redo-reset — a new command to clear all redo build state when the database becomes confused.

Problem

Sometimes redo's database can get confused (e.g., source files marked as targets, or stale state from incomplete builds) if a user performs actions outside of redo's purview. The only fix is rm -rf ~/.redo, which is hard to discover and easy to forget.

Solution

$ redo-reset
Resetting redo state... done.
  Removed /home/user/.redo (build state)
  Removed /tmp/redo-1001-locks (lock files)
  Removed /tmp/redo-1001 (temp/cache)
Redo state reset. Next build will start fresh.

What it removes

• ~/.redo/ — persistent build state (database, stamps)
• /tmp/redo-<user>-locks/ — lock files for parallel builds
• /tmp/redo-<user>/ — session temp dirs and caches

What it does NOT remove

• Build outputs

Design

• Implemented as a new symlink target (same pattern as redo-ifchange, redo-done, etc.)
• Handled early in main() before session initialization — avoids creating state it's about to delete
• Idempotent — running twice is safe (Nothing to reset.)
• --help and --version work as expected

Tests

11 tests in test/420-reset/all.do covering:

1. No prior state
2. State removal after build
3. Meta directory removal
4. Build works after reset
5. Build outputs preserved
6. Reports what was removed
7. Multiple resets idempotent
8. --help shows usage
9. Rebuild from scratch after reset
10. --version works
11. Output format