Rename fingerprint artifacts to expressions

[nahiyankhan]

Summary

• Rename the public Ghost artifact and API language from fingerprint to expression, with expression.md as the single canonical default.
• Replace parent/adopt tracking with neutral tracks / track, including the new sync manifest fields.
• Update docs, docs site copy, CLI help, schema generation, emitted skill/context bundles, tests, and the ghost-ui dogfood artifact.
• Rename observation.closestSystems to observation.resembles to avoid the design-systems-adjacent wording.

Breaking Changes

• fingerprint.md is no longer the advertised/default artifact; lint, describe, and emitted bundles now target expression.md.
• Public exports and internal module paths move from fingerprint-named APIs/modules to expression-named equivalents.
• adopt <source> is replaced by track <expression>.
• .ghost-sync.json now records tracks, trackedExpressionId, and localExpressionId instead of parent/child fields.

Verification

• pnpm check
• pnpm test (186 tests)
• pnpm --filter ghost-drift build
• node scripts/emit-expression-schema.mjs
• ghost-drift emit skill and emit context-bundle smoke checks produced expression-named files only

Release

• Adds a major Changesets entry for ghost-drift.