fix(models): retarget Composer aliases to Composer 2.5

[TiberiusNemesis]

Summary

Cursor retired the Composer 2.x model ids — cursor-agent --list-models now exposes only composer-2.5 and composer-2.5-fast. The plugin's composer / composer-fast / fast shortcuts still resolved to the dead composer-2-fast, and composer-full to composer-2, so those documented shortcuts handed Cursor an unavailable model id.

Retarget the short shortcuts to composer-2.5-fast and composer-full to composer-2.5, add explicit identities for the live ids, and keep composer-2 / composer-2-fast as passthrough aliases so users on older cursor-agent builds aren't broken. README, the cursor-runner agent guidance, command/package descriptions, and tests updated to match. Closes #8.

Test plan

• npm test — all 84 specs green
• npm run lint — prettier + eslint clean
• New tests added (live alias mappings + retired-id passthrough)
• Docs updated (README, CHANGELOG ## Unreleased, cursor-runner, descriptions)

Checklist

• No new runtime dependencies
• No build step, no new dist/ or .ts files
• User-visible behaviour change noted under ## Unreleased in CHANGELOG.md

[TiberiusNemesis]

@freema btw, I left version at 0.3.0 (didn't bump to 0.3.1) (see the ## Unreleased part in the doc), but I think the correct may be to bump? I dunno, your call here since you're the owner

[freema]

Verified end-to-end, thanks for the thorough report and fix:

• Confirmed against a live cursor-agent --list-models (macOS, 2026-06-11): only composer-2.5 and composer-2.5-fast (default) are listed — the 2.x ids are gone, so the composer/fast/composer-full shortcuts were indeed resolving to dead ids.
• Ran the branch locally: 84/84 tests green, prettier + eslint clean.
• Keeping the retired ids as identity passthroughs for older cursor-agent builds is the right call, and the docs sweep (README, agent doc, descriptions) is consistent.

One heads-up: this PR and #11 both add an ## Unreleased section to CHANGELOG.md, so whichever lands second needs a trivial rebase. I'll merge this one first.

Out of scope here, but noted for a follow-up: the grok alias (grok-4-20) is also retired — live list shows grok-4.3 / grok-build-0.1. Tracking that separately.