feat(score): display the trust tier alongside the numeric score (closes #236)

[dchaudhari7177]

Closes #236. Part of the #174 trust-tier roadmap.

mcp-observatory score printed only the number and letter grade, so the tier that turns it into a trust signal was invisible in the terminal.

  MCP Health Score: 95/100 (A)  Tier: Platinum
  MCP Health Score: 87/100 (B)  Tier: Gold
  MCP Health Score: 72/100 (C)  Tier: Silver
  MCP Health Score: 55/100 (F)  Tier: Bronze
  MCP Health Score: 30/100 (F)  Tier: Unrated

Notes

• Appended to the existing header, not added as a separate line, so the existing formatting is untouched as the issue asked.
• Tier colours are deliberately distinct from grade colours. Grade is green/yellow/red (pass/warn/fail); if the tier reused those, the two signals would read as one. Per the issue: platinum cyan, gold yellow; then silver blue, bronze and unrated dim. All from the existing ANSI palette in commands/helpers.ts, so --no-color keeps working.
• getTrustTier already exists in src/score.ts and is covered by tests/score.test.ts (including the 90/80/65/50 boundaries), so this PR adds only the rendering — no scoring logic changes, and formatTier is a pure lowercase→label helper.

Verification

npx tsc --noEmit          clean
npx vitest run tests/score.test.ts    15 passed

Rendered every tier band to confirm the line composes correctly with and without ANSI codes (the plain: forms above).

Note the repo's npm run lint crashes on this machine with TypeError: Cannot read properties of undefined (reading 'Cjs') — it does so on pristine main too, so it's environmental and not from this change.

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