docs: Enforce branch-based development in AGENTS.md

[i11v]

Summary

• Add Git Workflow section to AGENTS.md
• Enforce branch-based development with pull requests
• Never push directly to main

Test plan

• Review AGENTS.md changes

[github-actions]

JMAP Spec Coverage

effect-jmap@0.8.0 coverage:spec /home/runner/work/effect-jmap/effect-jmap
tsx scripts/spec-coverage.ts --markdown

Overall: 15/31 methods (48.4%)

Blob (0/4)

Method	Status
copy	⬚
get	⬚
lookup	⬚
upload	⬚

Core (0/1)

Method	Status
echo	⬚

Email (6/8)

Method	Status
get	✅
set	✅
query	✅
queryChanges	✅
changes	⬚
copy	✅
import	✅
parse	⬚

EmailSubmission (5/5)

Method	Status
get	✅
set	✅
query	✅
queryChanges	✅
changes	✅

Identity (0/3)

Method	Status
get	⬚
set	⬚
changes	⬚

Mailbox (4/5)

Method	Status
get	✅
set	✅
query	✅
queryChanges	✅
changes	⬚

SearchSnippet (0/1)

Method	Status
get	⬚

Thread (0/2)

Method	Status
get	⬚
changes	⬚

VacationResponse (0/2)

Method	Status
get	⬚
set	⬚