Add session prune command

[efenocchi]

Summary

• Add sessions prune CLI subcommand for scoped session cleanup by author
• Supports --all, --before <date>, --session-id <id> modes with --yes to skip confirmation
• Deletes both session rows (sessions table) and corresponding summary files (memory table)
• Only operates on sessions owned by the logged-in user

Test plan

• 174 unit tests passing (vitest)
• E2E claude -p --plugin-dir: capture creates sessions, sessions prune lists them, --all --yes deletes sessions + summaries
• Verified empty state message after full prune
• Tested against memory_test/sessions_test tables to avoid polluting production data

Closes #34

[claude]

Claude finished @efenocchi's task in 2m 2s —— View job

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• Read PR diff and changed files
• Analyze for bugs, security issues, correctness
• Post inline comments

Found 2 issues (1 bug, 1 minor):

• Bug extractSessionId: split("_").slice(3) returns wrong session ID when username/org/workspace contain underscores, causing summary files to be silently skipped during prune. Fix: use the same regex already in deeplake-fs.ts.
• Minor unused import: sqlLike is imported but never used.