docs: refresh stale documentation (6 files flagged)

## Problem

The `doc-gardening.cjs` script flags 6 documentation files as stale (not updated in 36-73 days):

| File | Days Since Update |
|------|-------------------|
| `docs/coding-conventions.md` | 73 days |
| `docs/conversation-memory.md` | 49 days |
| `docs/core-technologies.md` | 57 days |
| `docs/design.md` | 57 days |
| `docs/pull-requests.md` | 57 days |
| `docs/quick-start.md` | 36 days |

## Why It Matters

- Stale documentation misleads contributors and users
- The doc-gardening script warns about these files on every `npm run lint`
- Outdated conventions docs lead to inconsistent code style
- Outdated quick-start docs frustrate newcomers

## Proposed Action

Review and update each flagged file:

### 1. `docs/coding-conventions.md` (73 days)
- Verify TypeScript conventions match current `tsconfig.json`
- Check that Biome rules are documented
- Update any outdated patterns

### 2. `docs/conversation-memory.md` (49 days)
- Verify token budgeting logic matches current implementation
- Check that the rolling summary flow is accurate
- Update if the conversation memory feature has changed

### 3. `docs/core-technologies.md` (57 days)
- Update dependency versions
- Verify technology selection rationale is still valid
- Add any new dependencies

### 4. `docs/design.md` (57 days)
- Verify UI/UX principles match current implementation
- Update screenshots if the UI has changed
- Check that design tokens are current

### 5. `docs/pull-requests.md` (57 days)
- Verify PR process matches current workflow
- Update if review process has changed
- Check that branch naming conventions are current

### 6. `docs/quick-start.md` (36 days)
- Verify setup instructions work on a fresh clone
- Check that port numbers and URLs are correct
- Update Docker commands if they've changed

## Acceptance Criteria

- All 6 files are reviewed and updated
- `npm run lint` passes with 0 doc-gardening warnings
- Content matches current codebase behavior
- Links within docs are valid

Start by running the doc-gardening script via npm run lint, then review the six listed files: docs/coding-conventions.md, docs/conversation-memory.md, docs/core-technologies.md, docs/design.md, docs/pull-requests.md, and docs/quick-start.md. Compare their guidance with the current codebase, configuration, workflow, and setup instructions. Done means all six are accurate, internal links are valid, and npm run lint reports no doc-gardening warnings.

Written by the indexing model from the issue text.