feat(transports/axiom): add new Axiom transport

[theogravity]

Summary

Adds a new transport that ships structured logs to Axiom using the official axiom-go SDK.

What it does:

• Accepts a caller-supplied *axiom.Client (similar to gcplogging pattern)
• Serializes log entries as NDJSON and sends via Client.Ingest()
• Configurable message field key
• Supports all standard loglayer features: fields, metadata, errors, context

Files added:

• transports/axiom/ - Full transport implementation with tests and examples
  • axiom.go - Core transport with Config, New(), Build(), SendToLogger()
  • errors.go - ErrClientRequired, ErrDatasetNameRequired
  • axiom_test.go - 12 tests including contract tests
  • example_test.go - Usage examples
  • livetest_test.go - Live test against real Axiom API (build-tag gated)
• docs/src/transports/axiom.md - Documentation page

Configuration updates:

• monorel.toml - Register axiom module for releases
• .changeset/jolly-bear.md - Initial major release changeset
• AGENTS.md, CONTRIBUTING.md - Monorel installation, major version for new modules
• docs/.vitepress/config.ts - Sidebar entry under "Cloud"
• docs/src/transports/_partials/transport-list.md - Transport catalog entry
• docs/src/whats-new.md - Release announcement

Testing:

• All contract tests pass (go test ./transports/axiom/)
• Live test against real Axiom API passes with provided credentials
• Build and vet checks pass across all modules
• Docs build clean

Verification

# Tests
go test ./transports/axiom/
go test -tags=livetest ./transports/axiom/

# Build
bash scripts/foreach-module.sh build

# Live test (requires AXIOM_TOKEN and AXIOM_DATASET env vars)
AXIOM_TOKEN=<token> AXIOM_DATASET=test go test -tags=livetest ./transports/axiom/