chore: add agent context and refresh new chat icon (#268)

## Related Issue

Related to #251.

## Problem

Expert Talk decisions lacked repository-level domain context and agent
tracker guidance. The New Chat control also used inconsistent icons
across desktop and mobile navigation.

## What changed

- Add the canonical Expert Talk glossary and agent workflow guidance.
- Use the Remix New Chat icon across desktop and mobile navigation, with
hover motion and reduced-motion support from the global design system.
- Remove the obsolete custom conversation icon, add regression coverage,
and rebuild the committed Web bundle.
- Add the approved patch changeset.

Integration note: #265 also changes the icon registry and generated Web
assets and still uses pre-decision Expert Talk terms. Rebase and
reconcile #265 after this PR lands.

## Verification

- `pnpm test` — 20,863 passed; 3 expected failures; 79 skipped.
- `pnpm typecheck`
- `pnpm lint` — 0 errors; existing baseline warnings only.
- `pnpm build`
- `nix build .#pythinker-code`
- `pnpm check:web`
- `pnpm -C docs build`
- Browser checks at 1440 px light mode and 375 px dark mode.

## Checklist

- [x] I have read the
[CONTRIBUTING](https://github.com/PyModel/pythinker-code/blob/main/CONTRIBUTING.md)
document.
- [x] I have linked a related issue (external PRs: the issue must have a
maintainer's `/approve`).
- [x] I have added tests that prove my feature works.
- [x] Ran `gen-changesets` skill, or this PR needs no changeset.
- [x] Ran `gen-docs` skill, or this PR needs no doc update.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
  - Refreshed the New Chat icon across desktop and mobile navigation.
- Added a subtle hover animation to make interactive New Chat controls
feel more responsive.

- **Documentation**
- Added terminology and workflow guidance for Pythinker Code and Expert
Talk.
- Expanded guidance for domain context, issue tracking, and triage
labels.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: elkaix