Title: add --id for custom IDs and --dir for directory organization

[nthh]

Summary

Adds two features for better ticket organization:

• --id flag for custom ticket IDs
• --dir flag for organizing tickets into subdirectories
• --summary flag to show directory progress

Features

Custom IDs (--id)

tk create "My feature" --id my-new-feature
# Creates my-new-feature.md instead of auto-generated m-5c4a.md

Directory Organization (--dir)

# Create ticket in a directory (auto-creates if needed)
tk create "Pipeline v2" --dir pipeline-v2

# Filter commands by directory
tk ls --dir pipeline-v2
tk ready --dir pipeline-v2

# Show all directories with progress (sorted by most recent activity)
tk ls --summary

Example tk ls --summary output:
pipeline-v2          1/3 (33%)  [2 open, 0 in progress]
alpha-release        5/8 (62%)  [2 open, 1 in progress]
(root)               2/10 (20%) [7 open, 1 in progress]

Other Improvements

- ticket_path() now searches subdirectories for partial ID matching
- Ambiguous ID errors now show matching files for easier debugging

Use Case

These features enable organizing tickets by project/feature/release without requiring a separate "epic" abstraction - directories are just directories, and tickets can optionally live in them.