Add the strict private supervisor start request

[arasovic]

Summary

• add the private, schema-versioned supervisor start request document
• carry full prompts, binary data, write declarations, resolved model settings, roots, worktree metadata, and execution settings without argv or environment exposure
• reject unknown fields, trailing JSON, invalid UTF-8, malformed binary content, invalid run/task/write/time/worktree settings, and unsupported versions
• preserve prompts beyond the persisted 4 KiB projection cap and clone mutable request content at the boundary

Verification

• go test -race ./internal/background -run SupervisorStartRequest -count=20
• go test -race ./internal/background -count=2
• npm ci
• npm run verify
• go build ./...
• Windows test-package compilation
• two independent read-only reviews: no spec gaps and no actionable findings

Mutation proof

Focused tests failed as expected when deliberately changing each invariant in disposable worktrees:

• allow unknown fields
• accept trailing JSON
• collapse declared-empty writes into undeclared writes
• omit binary data content

No supervisor reply, role dispatch, process launch, Manager, CLI, docs, or release behavior is included.

Closes #204