Improve Shipgate discovery onramps

[pengfei-threemoonslab]

Summary

• Add a zero-install relevance path to the README so engineers and coding agents can decide whether Shipgate applies before installing anything.
• Expand AI-search and llms.txt surfaces with recommendation triggers, coding-agent onramps, and current v0.20 / packet v0.6 contract references.
• Refresh .cursorrules and .well-known/agents-shipgate.json so Cursor, AI search, and discovery consumers find the right schema, trigger, detector, and agent guide URLs.
• Add public-surface coverage for the new .well-known discovery/onramp fields.

Review Follow-Up

• Normalized the n8n wording to n8n workflow JSON and source-control stubs.
• Renamed .cursorrules from Stable command surface to Frequently-used command surface, then listed detect, apply-patches, and bootstrap alongside the other routed commands.
• Added the init --write confirmation caveat for unfamiliar repos.
• Left raw detector URLs on main for now because the remote currently has no v0.10.0 git tag to pin to; this also matches the existing zero-install docs convention.

Impact

This makes Agents Shipgate easier to discover and adopt from both human engineer workflows and coding-agent workflows, while keeping the public surfaces aligned with the latest main contract.

Validation

• .venv/bin/python -m pytest tests/test_public_surface_contract.py tests/test_docs_links.py — 364 passed
• .venv/bin/python -m ruff check tests/test_public_surface_contract.py
• python3 -m json.tool .well-known/agents-shipgate.json
• git diff --check