fix(npm): publish the launcher as @kerrshift/agentdiff

[lostmartian]

What

Renames the npm launcher from agentdiff to @kerrshift/agentdiff:

npx @kerrshift/agentdiff
npm i -g @kerrshift/agentdiff   # command is still `agentdiff`

Why

The v0.6.1 npm publish failed twice:

1. First failure: the token needed bypass-2FA. That's fixed now.
2. Second failure: npm itself rejects the name: 403 Forbidden - Package name too similar to existing package agent-diff.

agent-diff belongs to an unrelated product, a TypeScript SDK. npm also blocks the same name on PyPI-style similarity rules, so no token can unlock the unscoped name. A name under the kerrshift org scope isn't subject to that check, and the user's npm org is kerrshift, matching GitHub.

Nothing was ever published to npm under agentdiff, so no one is broken by the rename.

How

• npm/package.json: name → @kerrshift/agentdiff, plus publishConfig.access: public. Scoped packages default to private, and the workflow's --access public already covers this; the setting is a second safeguard.
• README install table and hero, npm README, launcher header comment, and publish.yml comments all switch to the scoped commands.
• tests/test_distribution.py asserts the scoped name, with a comment explaining why.
• CHANGELOG has a [Unreleased] / Fixed entry explaining that 0.6.1's npm publish never landed.

Publishing happens in a separate 0.6.2 release PR, per the release rule.

Testing

make lint                  -> All checks passed
uv run python -m pytest    -> 461 passed
cd npm && npm test         -> 4 passed
npm pack                   -> kerrshift-agentdiff-0.6.1.tgz
npm i ./kerrshift-agentdiff-0.6.1.tgz && npx agentdiff --since 7d   -> report renders (via uvx, PyPI 0.6.1)
npm publish --dry-run      -> "Publishing to https://registry.npmjs.org/ with tag latest and public access"

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