Cover the create-agent form hook: payload shaping, files, prefs

[selfcontained]

Summary

Recurring test-enforcer run. Local suite was green on first pass (13th consecutive run) — check ok; server 2366 pass / 8 skip, web 516, extension 60, scripts 4; inert E2E 175 pass / 12 skip; live terminal lane (pnpm run test:e2e:live, first post-#858 validation) 11 passed in 20.7s with zero leaked tmux sessions. CI scan clean — all recent failures are the two known branch-local issues (philosophy-site prettier, whiteboard migration-prefix guard).

With no failures or flakes to fix, this run adds the highest-value missing unit coverage: use-create-agent-form.ts, the create-agent dialog's entire state machine, had zero direct or indirect coverage while feeding the POST that creates real agents (worktree flags, branch fields, FormData switching).

New coverage — use-create-agent-form.test.tsx (25 tests)

• Agent type seeding: initialAgentType when enabled, first-enabled fallback, stored last-used type, and the re-snap effect when the current type drops out of the enabled list.
• Cwd seeding: trimmed resolveDefaultCwd, stored last-used fallback, ~/ default.
• Startup files/links: dedupe by name:size:lastModified, object-URL previews created only for images and only once per key, revocation on remove and on unmount, link dedupe.
• Paste routing: off-target ignore, files-win-over-text, URL text captured as a link (normalized + deduped), plain text left to the textarea.
• Submit payload shaping (the core value): exact JSON body on the config step; worktree suppression when the cwd is not a git repo; createNewBranch/worktreeBranch/baseBranch gating (trim, omitted-on-main); context-step prompt trimming and omission when blank; the JSON-vs-FormData switch including empty-field skipping (name dropped, fullAccess: false stringified); last-used localStorage writes + cwd history most-recent-first; toast error message vs. generic fallback; creating reset in finally.

Verification

• 8-mutant battery: 7/8 killed. The survivor (dropping the enabled-list guard in the createType initializer) is behaviorally equivalent at hook level — the re-snap effect corrects it within the same mount; the review agent confirmed the judgement.
• 25/25 pass deterministically, including with --sequence.shuffle.tests=true.
• Isolation hardening per repo rules: fresh jotai store per test plus an afterEach purge of the per-cwd createNewBranchPrefAtom family (atomWithLocalStorage bakes its initial localStorage read in at atom creation), explicit RTL cleanup(), explicit module-factory mock resets.
• Review agent: 0 must-fix, 1 should-fix (atom-family purge — applied), 4 nits (3 applied; skipped the importOriginal spread to stay consistent with the repo's existing minimal api mock factories).
• pnpm run check, full web suite (541), and pnpm run finalize:web all green.

Also filed DIS-167 for two unreachable branches in agent-card-header/status that need a product call (carried from the backlog).

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