feat: add --fit axis for seniority-tier composition

[danmestas]

Summary

Adds fit as a fourth composition primitive alongside outfit / cut / accessory. Fits express seniority tier (junior-engineer / engineer / senior-engineer / staff-engineer) and compose as a strict overlay between outfit and cut in the prose-union layer order: outfit → fit → cut → accessory. Skill/agent/hook union remains order-independent set algebra.

Closes #60.
Companion wardrobe PR: danmestas/wardrobe#122 (introduces the 4 fits content side).

What changed

• Schema: FitSchema mirrors CutSchema (overlay shape) with type: 'fit'.
• Discovery: new src/lib/fit.ts (findFit / listAllFits, 3-tier project → user → builtin); fits/ added to discover.ts; fit.md filename convention.
• CLI: --fit <name> wired through suit <harness>, suit up, suit prepare. Singleton — duplicate --fit flags error at parse time. Unknown fit names error with "Available: ..." list.
• Resolver: fit layers between outfit and cut for skill_include/exclude, include-block force-includes, enable/disable globals, and category intersection. cutPrompt unions fit body + cut body when both present; preserves cut-only output byte-for-byte for back-compat.
• Adapters (claude-code / codex / gemini / pi): emit [] for type: fit — same as cut/accessory, composed at resolve time.
• Lockfile: optional fit field on resolution block, omitted when null so pre-fit lockfiles remain bit-identical.
• Introspection: suit list fits and suit show fit <name>.

Test plan

• Unit: fits/<name>/fit.md resolves identically to a cut shape (FitSchema parsing, 3-tier discovery, project-scope wins, throws with available names listing)
• Unit: --fit <unknown> errors with available names
• Unit: duplicate --fit flags error as singleton violation
• Unit: fit.skill_include rescues a category-dropped skill
• Unit: fit.skill_exclude drops an outfit-included skill
• Unit: fit.include.skills force-rescues a skill
• Unit: outfit → fit → cut prose unioning in cutPrompt
• Unit: progressive category intersection across outfit + fit + cut
• Integration (against /Users/dmestas/projects/wardrobe): SUIT_CONTENT_PATH=$WD suit list fits → 4 fits returned
• Integration: SUIT_CONTENT_PATH=$WD suit show fit engineer → renders manifest + body
• Integration: SUIT_CONTENT_PATH=$WD suit prepare --outfit code --fit junior-engineer --target claude --dry-run → 49 deduped files emitted
• Integration: SUIT_CONTENT_PATH=$WD suit up --outfit code --fit junior-engineer --cut executing writes lockfile with fit: junior-engineer
• Local CI mirror: npm ci && npm run typecheck && npm run build && npm link && npm test — 584/584 pass (557 existing + 27 new)

Back-compat

• RunUpArgs.fit and RunPrepareArgs.fit are optional — pre-fit callers compile unchanged.
• Lockfile resolution block omits fit when null — existing test fixtures bit-equal.
• cutPrompt preserves cutBody ?? '' byte-for-byte when only cut is supplied.
• ManifestSchema discriminated union adds FitSchema as a new branch — non-fit manifests parse unchanged.
• v1 lockfile schema accepts both with-fit and without-fit shapes.