feat: show session name on HUD line 1

[staryxchen]

What

Append the current session name as a trailing segment on HUD line 1, so the status line reads e.g.:

[Claude-Sonnet-4.6-1M] │ my-project  git:(main*) │ session: PR#30777
Context ████░░░░░░ 41.8%
✓ Bash ×32  ✓ Read ×8  ✓ Edit ×5  ✓ WebFetch ×3  ✓ Grep ×2

The session: <name> segment is omitted entirely when the session has no title, so existing sessions render unchanged.

Why

CodeBuddy Code tracks a per-session title (set via the built-in /rename action, or auto-generated as an ai-title), but this information wasn't surfaced in the HUD. Showing it lets you tell sessions apart at a glance in the status line.

How

New get_session_name(transcript_path) in hud.py resolves the name from the same transcript already read for tool stats, but via a separate grep -E 'custom-title|ai-title' pass rather than the 800-line tail window — a rename can fall outside that window in a long session, and title entries are sparse so a full-file grep is cheap.

Resolution matches the UI's getCustomTitle semantics:

• Return the last custom-title entry's customTitle (set via /rename).
• Fallback to the last ai-title entry's aiTitle (auto-generated).
• Return '' when neither exists → the line-1 segment is omitted.

As with the rest of the script, any failure (missing file, grep timeout) returns '' — the status line never crashes.

Coloring

The session: label is CYAN, matching line-1 structural labels (folder, git:). The name is MAGENTA, keeping it visually distinct from the YELLOW branch inside git:(...).

Tests

• 8 unit tests for get_session_name: missing file, empty transcript, no title entries, custom-title returned, last custom wins, ai-title fallback, custom preferred over ai, empty custom falls back to ai.
• 3 end-to-end tests: line 1 contains session: <name>; line 1 omits the segment when unnamed; ai-title shown end-to-end.

Full suite: 41 passed.

$ pytest tests/ -q
.........................................                                [100%]
41 passed in 0.30s

Notes

• grep is added to README Requirements (the script now shells out to git, tail, and grep).
• No new stdin fields — the statusLine payload has no name/title key (only an opaque session_id), so the name is derived from the transcript.
• Verified against real transcripts: a custom-title session shows │ session: PR#30777; an ai-title-only session shows the auto-generated title; unnamed sessions are unchanged.