docs: establish pattern for capturing unwritten assumptions

## Context
Need a lightweight way to capture emerging assumptions and patterns before they're ready for formal documentation. As discussed: "Q&A-based documentation is more important than it all being in one place."

## Problem
Assumptions emerge during development but aren't always ready for the knowledge base. We need a low-friction way to capture these insights without cluttering the main codebase or going through PR process for every thought.

## Three Competing Approaches

### 1. Local ASSUMPTIONS.md (gitignored)
```bash
# In .gitignore
ASSUMPTIONS.local.md

# Usage
$EDITOR ASSUMPTIONS.local.md # Quick local edits
```
**Pros**:
- Zero friction, instant updates
- Private experimentation space
- No PR process

**Cons**:
- Not backed up
- Can't share with team
- Lost on machine switch

### 2. GitHub Wiki
```bash
gh repo edit --enable-wiki
git clone https://github.com/atxtechbro/dotfiles.wiki.git wiki
```
**Pros**:
- Version controlled
- No PR process
- Accessible from anywhere

**Cons**:
- Separate from main codebase
- AI agents might not discover it
- Another thing to maintain

### 3. Syntactic Prefix Comments
```bash
# ASSUMPTION: Q&A-based docs > single source of truth
```
```python
# TODO(assumption): This pattern assumes AI agents will be primary readers
```
```
git commit -m "feat: add search" -m "Assumption: breadcrumbs > hierarchy"
```
**Pros**:
- Lives with the code
- AI-discoverable in context
- Shows up in git history

**Cons**:
- Scattered across codebase
- No central list
- Can clutter code

## Recommendation
Start with **syntactic prefix comments** as the primary approach:
- Use `# ASSUMPTION:` for inline documentation
- Use `Assumption:` git commit trailers for commit-level insights
- Keep assumptions close to where they apply

If we need centralization later, we can grep for `ASSUMPTION:` patterns and extract them.

## Examples from Current Codebase
- OSE principle: "Grant broad tool permissions - just as managers don't want employees asking permission to staple papers"
- Permission philosophy: Now in knowledge base after starting as JSON comments
- Q&A documentation: This very assumption about modern workflows

What approach resonates with the team?

Principle: versioning-mindset

No repository file, test, or entry point is identified. Review the three proposed approaches and the recommendation in the issue, then determine whether the team has chosen a convention. Done means the decision and resulting assumption-capture pattern are documented and agreed upon.

Written by the indexing model from the issue text.