G5A: improve minimal public discovery surface

[davidqyc]

Implements the G5A minimal public discovery surface for davidqyc/codex-preserve (Issue #3).

What changed (exactly two files)

README.md — public install/discovery repair

• Primary public install command is now python -m pip install codex-preserve (was the local-source pip install .).
• The Install section now sits near the top and gives a short first-use path: install → codex-preserve --help (safe read-only surface).
• Added compact navigation to the public project surfaces (PyPI, Releases, Issues) as inline links — no badge wall, no promotional language.
• The synthetic-examples section is reworded so it no longer implies ./examples/run_examples.sh ships in the installed wheel; it is stated as a repository-checkout surface.
• The built-in codex archive distinction, the non-affiliation wording, and the manifest-relative integrity limitations are all unchanged and not strengthened.

pyproject.toml — durable project URLs for future package metadata

[project.urls]
Homepage = "https://github.com/davidqyc/codex-preserve"
Repository = "https://github.com/davidqyc/codex-preserve"
Issues = "https://github.com/davidqyc/codex-preserve/issues"
"Release notes" = "https://github.com/davidqyc/codex-preserve/releases"

Explicitly unchanged

• No product behavior change; all five runtime modules (exporter.py, verify.py, cli.py, __init__.py, __main__.py) are byte-identical to the base tree (SHA-256 verified).
• No version bump: 0.1.0 remains. No tag, no GitHub Release, no PyPI publish.
• No dependency, classifier, script, license, or build-backend change.
• .github/workflows/* unchanged.

Local validation

• python3 tools/public_hygiene_scan.py . — PASS (0 findings)
• PYTHONPATH=src python3 -m unittest discover -t . -s tests — PASS (277 tests)
• ./examples/run_examples.sh — PASS (0/1/2 verdicts as documented)
• python3 -m build — PASS (sdist + wheel)
• python3 -m twine check --strict dist/* — PASSED for both artifacts
• Built metadata: Version: 0.1.0, License-Expression: Apache-2.0, all four Project-URL entries present.

Closes nothing; reference: Issue #3. Not to be merged in G5A — Coordinator review required.