chore(repo): update canonical URLs for kerrshift org transfer

[lostmartian]

What

All hardcoded github.com/lostmartian/agentdiff URLs now point to github.com/kerrshift/agentdiff: pyproject.toml Homepage/Repository metadata, README CI badge + clone link, the baseline-envelope schema $id, the docs link in agentdiff init-generated workflows, and CHANGELOG release links. Adds a [Unreleased] changelog entry.

Why

The repository was transferred to the kerrshift organization (repo transfer, GitHub App migration, PyPI trusted publisher, and Cloudflare Pages reconnection already completed). Old URLs currently rely on GitHub's automatic redirect, which silently breaks if a repo is ever created at the old path — canonical references should point at the new location.

How

• Mechanical lostmartian/agentdiff → kerrshift/agentdiff sweep across the five product files (negative lookahead keeps lostmartian/agentdiff-demo references intact — the demo repo still lives on the personal account).
• Reusable-action reference pattern is now kerrshift/agentdiff/.github/actions/agentdiff-check.
• Package metadata URLs refresh on the next PyPI publish (no code behavior change; no schema-version bump — the agent_trace.schema.json contract is untouched).

Testing

• make lint → All checks passed
• uv run python -m pytest → 429 passed

Checklist

• Exactly one logical change in this PR
• make lint passes
• uv run python -m pytest is green
• Docs updated if user-facing
• CHANGELOG.md entry added under [Unreleased] (if feat/fix)
• Conventional title: type(scope): summary

Roadmap link: n/a — housekeeping for the org transfer (WS-H distribution).

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