docs: correct top-level documentation to the shipped behaviour

[justin13888]

Summary

README, RELEASING, AGENTS, the crate overview in src/lib.rs, and the comments in ci.yml and hk.pkl now say only what the repository does today. This PR changes documentation and comments only. No code, workflow step or hook step changes.

Closes #129

Changes by path

• README.md
  • Drift: a managed file you edited by hand is a conflict that plan and apply report and apply skips. There is no keep/discard/skip prompt; the only prompt is the single yes/no confirmation in src/command.rs.
  • doctor: lists the nine checks in src/doctor/mod.rs. The claims about caches at their limit, integrations gone stale, and tools installed but not reachable are gone, because no such checks exist.
  • Removes "and tells you what it saved", which no output backs.
  • Documents bx self-upgrade --force, the 2 exit code of --check, the PATH argument that bx add and bx rm require (without one they refuse and point to bx init), and their 2 exit codes.
  • Lines 3 and 14 are unchanged.
• RELEASING.md
  • Describes the real chain: the release job's releases_created output gates build, and upload-assets attaches the tarballs and .sha256 files.
  • Recovery says to re-run the failed jobs, because a full re-run finds nothing new to release.
  • Removes the "first release" section, since v0.1.0 and v0.1.1 already exist.
  • Describes the one optional secret in prose instead of a "neither/both" table with one row.
• AGENTS.md: is now a symlink to CLAUDE.md. The two copies had diverged, and AGENTS.md was missing the sentence about cached tool activation output.
• src/lib.rs: the doctor sentence in the crate overview now names all nine checks. It previously named seven and left out references and orphans.
• .github/workflows/ci.yml: the comments on the uv pin, the action-ref resolver and its self-check now state current intent in a few lines each. Removed: review-round history ("until round 5", "over-correction", "survived a round"), line-number citations into another repository, and the claim that checkout is "the one ref" this step cannot catch. The last line now says it cannot report a broken ref in this job's own setup. No step changed.
• hk.pkl: the header now lists the commit-msg hook (convco) and the pre-commit shellcheck, actionlint and line-check steps.

Decisions taken

1. Doc-vs-code contradictions in README (drift prompt, doctor checks, 'saved')
   Taken: the documentation is wrong; it is corrected to the shipped behaviour, and the unbuilt capabilities are filed separately without the label (accepted unrebutted)
   Rejected: the code is wrong - building them would be a feature, out of scope for a cleanup
   Reverses: restore the removed README sentences
2. How to stop AGENTS.md and CLAUDE.md diverging
   Taken: AGENTS.md becomes a symlink to CLAUDE.md (accepted unrebutted)
   Rejected: copy the missing sentence across - they would diverge again
   Reverses: replace the symlink with a copy
3. Where to document the exit codes of add, rm and self-upgrade --check
   Taken: one paragraph under the existing "Exit codes" table, which already documents plan's codes
   Rejected: one table per command - it repeats the same four codes
   Reverses: move the paragraph
4. RELEASING recovery instruction
   Taken: "re-run that run's failed jobs" - a full re-run starts the release job again, which finds nothing new to release, so build and upload-assets are skipped
   Rejected: keep "re-run the workflow"
   Reverses: restore the previous sentence

Validation

• The pre-push hook passed at each push: mise run format-check, mise run lint, mise run test and mise run coverage.
• mise run actionlint and mise run line-check are clean. The pre-commit hook also ran convco, shellcheck, actionlint and line-check.

Coverage gaps

• Nothing tests prose, so the README and RELEASING statements are checked only by reading them against src/main.rs, src/command.rs, src/adopt.rs, src/upgrade.rs, src/doctor/mod.rs and .github/workflows/release-plz.yml.
• Nothing checks that AGENTS.md stays a symlink.

Follow-up

Decision 1 says the unbuilt capabilities are filed separately without the label. They are in #146: the drift prompt and the doctor cache and staleness checks. The removed "tells you what it saved" claim and the "installed but not reachable" doctor wording describe nothing planned, so they were removed with no follow-up.