feat(sdk): add structured headless agent adapters

[kjgbot]

Refs #141.

Outcome

• Adds a closed SDK-owned HeadlessAdapter registry for declared Claude, Codex, and Grok CLIs.
• Delivers provider instructions only by stdin (Claude/Codex) or a private temporary prompt file (Grok); no instruction reaches argv.
• Parses provider final text, structured trajectory, normalized usage, session id, and available Claude subagent evidence.
• Completes malformed or final-message-free zero-exit provider output as worker_error; preserves the hardened feat(sdk): declare agent CLI and model with fail-closed checks #136 custom-wrapper session unchanged.
• Reuses provider invocation definitions for model readiness preflight and execution.

Verification

$ ./node_modules/.bin/tsc && node scripts/make-cli-executable.mjs && RELAYFLOWD_BIN=/Users/khaliqgant/.relayflows-toolchain/target/1914866954/debug/relayflowd RELAYFLOWS_ALLOW_ANALYZER_SKIP=1 ./node_modules/.bin/vitest run --reporter=dot --maxWorkers=1 --minWorkers=1

Test Files  22 passed | 1 skipped (23)
     Tests  349 passed | 4 skipped (353)
Duration  155.75s
exit: 0

The four skips are the three explicit installed-provider opt-in cases and the real analyzer availability case; all fixture and live-daemon coverage passed.

$ tar -czf /tmp/flows-141-rebased-pack.roLvtS/relayflows-sdk.tgz package.json dist src && tar -tzf /tmp/flows-141-rebased-pack.roLvtS/relayflows-sdk.tgz | rg '^(dist/(index|headless-adapter)\\.(js|d\\.ts)|src/headless-adapter\\.ts)$'\ndist/index.js\ndist/headless-adapter.js\ndist/headless-adapter.d.ts\ndist/index.d.ts\nsrc/headless-adapter.ts\n\n$ node --input-type=module <built artifact smoke>\n{"finalText":"packed-final","trajectory":[{"type":"result","result":"packed-final","usage":{"input_tokens":1,"output_tokens":2}}],"usage":{"tokens_in":1,"tokens_out":2,"dollars":"0"}}\nexit: 0\n```\n\nNo merge or publish performed.

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