Correct MCP trust fingerprint, store path, and auth UX

[TheGreatAxios]

Summary: Applies verified CL-7899 doc fixes W1–W3 to docs/MCP.md (docs text only, no behavior change). W1: trust fingerprint covers sorted env key names, never values; type defaults http-when-url-else-stdio. W2: store path ~/.corbits/trust/.json, first 32 hex of SHA-256 of realpath-canonicalized root. W3: first-run auth is the mcp ! prompt-box mark; Enter in /mcp opens the URL and auto-copies the link. W4/M1 skipped per spot-check against code: no root PLUGINS.md exists (only docs/PLUGINS.md), and the briefed slug rule matches session/project-key.ts slugSegment, not mcp/auth-store.ts serverDisplaySlug — verifier should re-check those two. Verification: bun run check exit 0 (7307 pass, 0 fail). Fixes CL-7899

[linear-code]

CL-7899

[TheGreatAxios]

M1 applied (cd9e758): docs/MCP.md now states the exact auth-slug rule per src/mcp/auth-store.ts:42-48 (serverDisplaySlug via legacyServerSlug). W4 decided no-change: docs/PLUGINS.md left untouched. No merge.

[TheGreatAxios]

Critic review — docs-only, merge-ready

Spot-checked every new claim against code. All accurate.

Verified

• docs/MCP.md trust fingerprint (sorted env key names, never values; type defaults http-when-url-else-stdio) matches mcpServerFingerprint in src/trust/project-trust.ts:289-299.
• Store path ~/.corbits/trust/<repo-hash>.json, first 32 hex of SHA-256 of realpath-canonicalized root, matches projectTrustPath in src/trust/project-trust.ts:102-109 (SETTINGS_DIR_NAME=".corbits" per src/branding.ts:24).
• First-run auth UX (mcp ! prompt-box mark, Enter in /mcp opens URL + auto-copies, SSH rationale) matches src/tui/command-surfaces.ts:1538-1555 (openAuthURL + writeClipboard with SSH-fallback comment) and src/tui/runner/mcp.ts:195; mcp ! mark confirmed in src/tui/shell/internals.ts:464-465. Existing test src/tui/command-surfaces.test.ts:1267 covers Enter-opens-and-copies.
• Slug rule ([^A-Za-z0-9_-] → _, case preserved, empty → server, truncate 48) matches legacyServerSlug/serverDisplaySlug in src/mcp/auth-store.ts:42-48.

Blocking: none.

Nits (non-blocking)

• Placeholder drift: this PR renames to <repo-hash> here, but docs/PLUGINS.md:25 still says ~/.corbits/trust/<cwd-hash>.json. Consider aligning in a follow-up.
• projectTrustPath hashes the caller-passed cwd after resolve+realpathSync (with lexical fallback), not strictly a git toplevel — "project root" reads fine in practice but is slightly idealized.

Hygiene: docs text only, 1 file, +11/−8, no behavior change. All CI checks green (build, typecheck, eslint, prettier, tests, static-analysis).

Verdict: approve — safe to merge.