feat: add --last-accessed flag to expose tab.lastAccessed timestamp

[kevintraver]

Summary

• Add opt-in --last-accessed / -L flag to bt list and bt query commands
• Exposes Firefox's tab.lastAccessed timestamp (milliseconds since epoch)

Changes

• Extension: Always includes lastAccessed as 4th field in tab output
• Python CLI: Strips 4th field by default for backward compatibility
• Tab class: Parses lastAccessed when present, defaults to 0

Usage

# Default (3 fields, unchanged):
bt list
# a.1.123	Page Title	https://example.com

# With --last-accessed flag (4 fields):
bt list --last-accessed
# a.1.123	Page Title	https://example.com	1768780769297

Test plan

• bt list returns 3 columns (backward compatible)
• bt list --last-accessed returns 4 columns with timestamp
• bt query returns 3 columns (backward compatible)
• bt query --last-accessed returns 4 columns with timestamp
• All existing tests pass

🤖 Generated with Claude Code