Add AGENTS.md with coding and commit rules

AGENTS.md

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+# AGENTS.md — Rules for AI Agents
+
+## Before Every Commit
+
+1. **Format**: `gofmt -s -w .` and `goimports -w .`
+2. **Lint**: `golangci-lint run --config .golangci.yml ./...` — zero issues
+3. **Test**: `go test -race ./...` — all passing
+4. **Build**: `go build ./cmd/chatd` — compiles clean
+
+No commit lands on main with lint errors, test failures, or formatting issues.
+
+## Code Style
+
+- Follow CONVENTIONS.md exactly — it defines the architecture
+- All exported types, functions, methods, and vars must have doc comments
+- All packages must have a package doc comment
+- JSON tags use camelCase (not snake_case)
+- No magic numbers — use named constants
+- No inline error handling (`if err := ...; err != nil`) — use plain assignment
+- Blank line before every `return` statement
+- Blank line separating embedded fields from regular fields in structs
+- Blank imports must have a justification comment
+
+## Git Workflow
+
+- Never commit directly to main — always feature branches + PRs
+  - Exception: initial scaffolding and AGENTS.md/CONVENTIONS.md updates
+- PR titles: `Description (closes #N)` when closing an issue
+- One logical change per commit
+- Commit messages: imperative mood, concise
+
+## Architecture
+
+- Dependency injection via Uber fx — all components are fx providers
+- `internal/` for all application packages — one package per concern
+- Handlers return `http.HandlerFunc` using the closure pattern
+- Request/response structs defined inside handler closures
+- Models embed `models.Base` for database access to relation methods
+- Database migrations in `internal/db/schema/` — embedded via `go:embed`
+- SQLite by default (pure Go driver: modernc.org/sqlite)
+
+## Testing
+
+- Table-driven tests preferred
+- Use `testing.T` helpers, not assertion libraries
+- Test files live next to the code they test
+- `_test.go` suffix, same package for white-box testing
+
+## Dependencies
+
+- Required libraries are listed in CONVENTIONS.md §1
+- No new dependencies without justification
+- Pure Go preferred (no CGO)