feat: add an installer that installs and updates the pack safely

[TKontu]

Adoption used to mean copying files by hand, with no way to update them afterwards. As a result, existing adopters' copies have drifted. This PR adds scripts/install.py (standard library only):

python3 path/to/Reconcile-loop/scripts/install.py path/to/project [--claude] [--codex] [--dry-run]

• It copies the core files: SPEC.md, .agent/commands/, the config example, scripts/round.py, templates/ and the example. The MIT notice goes to .agent/LICENSE. Relative paths are preserved, and .agent-runs/ and HANDOFF.md are added to .gitignore.
• .agent/pack.lock records the pack version (from git describe), the selected runners and the SHA-256 of each installed file.
• Running it again from a newer pack is an update:
  • Files that are unmodified relative to the lock are replaced.
  • Locally edited or pre-existing project files are kept, and the new version is written beside them as *.pack-new.
  • Locally deleted files stay deleted.
  • Files dropped from the pack are removed only if unmodified.
• It refuses a dirty checkout, a target that isn't a repository root, and the pack itself. --dry-run writes nothing.
• It never writes the project README, AGENTS.md or .agent/config.toml; merging those stays with bootstrap.
• README and bootstrap now point at the installer, and copying by hand is still supported.

Validation:

• RED: the 8 new installer tests failed before implementation. Two follow-up behaviors were each added test-first: locally deleted files stay deleted, and --dry-run works on a dirty checkout.
• GREEN: python3 -m unittest discover -s tests passes (32 tests on this branch). This includes a check that all markdown links resolve inside an installed project. git diff --check is clean.
• Dry runs against two existing adopters reported 26 files to add in each. Their six project-specific phase wrappers were reported as conflicts to be written as *.pack-new, not overwritten.

Not included: release tags, a template repository, a plugin or a PyPI package. Those can wrap this same installer later. This PR is independent of #5; once #5 merges, its new files are picked up automatically through the glob patterns.

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