feat(): session and event identification

[StuBehan]

Summary

Sessions become first-class identities: renames persist, propagate into the Events tab live, and every row carries a stable color accent so the eye can link a nudge to its session.

Changes

• Persistence — new panel/SessionPersistence.swift. Disk-backed store at ~/.stack-nudge/sessions.json, keyed by (canonical-agent, projectPath). Atomic write, decode-failure-safe. Agent.canonical(_:) bridges wire names (claude-code, cursor → claude) so events and sessions agree on identity.
• SessionStore — seeds customName from persistence on every newly-discovered session (survives pid churn + restart). Writes on rename. Bumps lastSeenAt once per launch per named session, not per poll.
• EventRow — @EnvironmentObject SessionPersistence injected at the panel root. The "· project" pill now shows customName when set, else project basename. Render-time lookup, so a rename in the Sessions tab updates matching event rows instantly.
• Sessions tab — redesigned as a two-line card: name + agent tag + status (active · 14:23 / ended 5m ago); terminalApp · tilde-abbreviated cwd. Selected cards expand with N nudges · last X ago for active events matching the session.
• Color identity — SessionColor deterministic FNV-1a over the canonicalized join key → palette of 6 muted colors. Applied as a 3pt left-edge accent on both SessionRow and EventRow. Active 0.85 opacity, finished 0.45.
• Tests — 30 new XCTest cases across SessionPersistenceTests, AgentTests, SessionColorTests. Cover roundtrip, removal semantics, decode-failure recovery, canonicalization on read/write, determinism, palette membership.
• build.sh — added SessionPersistence.swift to the explicit source list (SPM auto-discovers; build.sh doesn't).

Testing

• swift build and ./build.sh clean.
• make reload, fired varied trial events: same-project events share a color, different projects get different colors, gemini gets its own, long titles still truncate, no-projectPath events render without a session chip.
• Rename loop end-to-end: renamed a running claude session in the Sessions tab → matching event rows picked up the new name immediately.
• 30 new XCTest cases — CI's test-macos job runs them; not run locally (CLI tools only, no Xcode).