chore: add AGENTS.md, git workflow rule, and expand README architecture

[gisk0]

Why

Postmortem from building the App Store subsections feature. Key friction points that docs can fix:

• Finding the project: multiple search timeouts before locating it
• Understanding architecture: had to cold-read 5 source files before writing code
• Git rule violation: pushed directly to main (no cursor rule enforcing PRs)
• Unused import on push: trunk caught it on push; should catch pre-commit

What's Added

.cursor/rules/git-workflow.mdc (alwaysApply: true)

Enforces branch + PR workflow. Never push directly to main.

AGENTS.md

Full code guide for agents and developers:

• Data flow diagram (DataSource → orchestrator → Telegram)
• Key files table with one-line descriptions
• All sources with what they fetch
• BriefingItem rendering reference (sentimentPrefix, monospace, url patterns)
• How to add a new source (including the mock pattern)
• Local dev commands
• Test coverage map (source file → test file)
• Reminder to run trunk before committing, not just before pushing

README.md

Expands Architecture section from infrastructure-only to include code structure tree and data flow summary.