feat(trail): add `entire trail tune` to tailor runner prompts to the repo

entire trail tune [<runner>] gathers signal about the current repo across four best-effort, gracefully-degrading tiers — repo docs/structure, merged PRs and issues (via gh), checkpoint churn hotspots, and past trail findings — and produces a prompt that rewrites the .entire/runners/*.json templates so their dimensions and score bands fit this repo instead of the generic defaults.

By default it prints the prompt for pasting into an agent. With --run it executes the prompt headlessly through the configured summary provider and surgically rewrites only each runner's prompt.template via byte-level replacement, leaving all other fields and formatting byte-for-byte intact (minimal git diff; files are git-tracked so the user reviews via git diff).

The CLI has no runner struct/loader (the backend consumes these files and substitutes {{placeholders}}), so runners are treated as opaque text.

Unit tests cover output parsing, surgical template replacement, runner loading/filtering, source-flag parsing, and prompt assembly.