[Plugin] adding install id to the active-session marker

[alexfarnhambrown]

Summary

Adds the installation UUID to active-session.json so Vercel CLI telemetry can attribute a command to the plugin installation that ran it.

#136 added plugin:install_id to the plugin's own phone-home but deliberately kept it out of the marker. The CLI only reads the marker, so today it can tell that a command ran inside a plugin session but not which installation that session belonged to. Consumer PR: vercel/vercel#17542. Tracked in PIPE-6602.

Changes

• hooks/src/telemetry.mts — installId?: string on ActiveSessionMarker, populated from the existing getOrCreateInstallationId()
• README.md — the telemetry section stated the ID is never written to the marker, which this changes
• Version 0.48.1 → 0.49.0, hook bundle rebuilt
• Test coverage for the new field

Stays on schema 1

The field is additive and optional, so older readers ignore it. A bump would actively break things: the shipped CLI reader does a hard marker.schema !== 1 → return null, so schema 2 would blank out vercel_plugin_active_session and vercel_plugin_version for everyone until they upgrade their CLI.

Optional rather than required because getOrCreateInstallationId() returns null when the file can't be written or read back.

Tests

bun test          # 975 passing
bun run validate  # clean

Rollout note

anthropics/claude-plugins-official pins this repo to commit 19606ac (0.45.1), 10 commits behind main. #136 is inside that gap, so the official-marketplace population isn't emitting plugin:install_id yet and won't pick this up either until that pin moves.

[jeffsee55]

There was earlier discussion about whether this UUID should stay plugin-only or also go on active-session.json #136 kept it plugin-only so as long as we're ok to do that 👍