feat(countdown): Add countdown app with show, search, create, edit, comment

[holmboe]

Summary

Add new countdown module for interacting with Phorge/Phabricator countdowns (C123 monograms).

Features

• show: Display countdown details with remaining time
• search: Search by text query or author
• create: Create countdowns with ISO 8601 or relative epoch (+7d, +2w)
• edit: Modify title, epoch, description, tags, subscribers
• comment: Add comments with CLI, stdin, or $EDITOR

Monogram shortcuts

• phabfive C1 → countdown show C1
• phabfive C1 "text" → countdown comment C1 "text"

Output formats

Supports rich (default), yaml, json, and tree output formats.

Known Issue

Creating or editing countdown epoch via Conduit API currently fails due to an upstream bug in Phorge:

• https://we.phorge.it/T16641

The code is written correctly and will work once the upstream bug is fixed. All other operations (show, search, edit name/description/tags/subscribers, comment) work correctly.

Test plan

# Show countdown
phabfive countdown show C1
phabfive --format=yaml C1

# Search
phabfive countdown search "launch"
phabfive countdown search --author=@me

# Edit (name, description, tags work; epoch blocked by upstream bug)
phabfive countdown edit C1 "New Title"
phabfive countdown edit C1 --description="Updated"

# Comment
phabfive countdown comment C1 "Looking forward to this!"
phabfive C1 "Quick comment"

🤖 Generated with Claude Code

[holmboe]

⚠️ Blocked on upstream bug: https://we.phorge.it/T16641

The countdown create/edit epoch functionality will not work until this Phorge bug is fixed. The bug is in Phorge's Conduit API where the epoch transaction type doesn't properly convert integer timestamps to the expected datetime object.

This PR should not be merged until T16641 is resolved upstream.