prompts: add Sashiko review profile

[OllieinCanada]

Summary

• add a bundled prompt profile for reviewing Sashiko's Rust code and service boundaries
• load an optional project-context.md in every review stage
• keep existing kernel, systemd, and iproute prompt contexts unchanged when that optional file is absent
• add deterministic bundle/loading/stage tests with no model or external-service calls

This is the next independently reviewable step toward “Sashiko for Sashiko.” The existing review binary can select the profile with --prompts third_party/prompts/sashiko. Daemon prompt-directory selection remains separate plumbing in #447; this PR neither depends on that PR nor changes daemon configuration.

Safety and scope

The only runtime plumbing change is one optional local-file load in PromptRegistry::build_context(). Missing project-context.md remains a no-op, which preserves the generated shared context for every existing profile. The new Sashiko profile does not enable itself automatically.

The profile is based on concrete Sashiko boundaries: Tokio task/channel lifecycle, Git and worktree ownership, subprocess cleanup, webhook authentication and repository URL safety, persistence/retry idempotence, provider error handling, token budgets, secret redaction, and configuration compatibility.

This PR does not change forge ingestion, NNTP, Git operations, database schemas, AI providers, review output, kernel prompts, or multi-repository behavior. It adds no dependencies and performs no remote prompt loading.

Deterministic validation

• Sashiko profile bundle/loading/stage tests: 3 passed
• optional project-context compatibility tests: 2 passed
• cargo fmt --all -- --check: passed
• cargo clippy --all-targets --all-features --release -- -D warnings: passed
• git diff --check: passed
• GitHub Actions on native Linux: DCO, lint, and Rust unit-test jobs passed
• no AI provider, paid model, external service, daemon, or integration test was invoked