bravebot profile: run Docker platform gates only for a story about another platform

[netzenbot]

User impact: none for a person using bravebot -- this changes which checks the loop runs before it opens a bravebot PR.

The problem

For every bravebot story that changed Rust, the loop's last validation step ran check-msrv, check-windows and check-linux. Each copies the whole worktree into a Docker container and builds it cold, so every iteration spent a long time re-checking platforms the host does not run. make check-affected already checks the host, and bravebot's CI checks the other platforms.

Reproduce

1. In this repo, run python3 -c "import json; print(json.load(open('projects/bravebot/profile.json'))['validations'][-1])" on master.
2. The step tells the agent to run the Docker gates for any change to Rust, with no exception for a story that has nothing to do with another platform.

The fix

The step now says to run the Docker gates only when the story is specifically about a platform other than the host's: the issue names Linux, Windows or an architecture, or the diff changes code gated to one. Otherwise the agent skips them and records Docker gates skipped: host-only in progress.txt. projects/bravebot/docs/testing.md says the same. A new test pins the wording. This is the bravebot profile, so it applies in every PRD mode.

Test plan

• python3 -m pytest tests/test_scripts.py -k TestBravebot -- passed
• make lint -- passed
• make test -- one unrelated failure, TestReviewRequestQueue (wc -l pads its count on macOS)
• CI passes cleanly