feat: select notes agent profiles directly

[yattdev]

Overview

Switch the Notes plugin's Enhance with AI agent selection from the legacy utility_agent format to a direct agent_profile format, while preserving backward-compatible error handling for existing utility-agent configurations.

Changes

• manifest.yaml: Replace utility_agent config key with agent_profile (format: agent-profile), keep agent_invoke capability, bump version to 0.5.0
• server/plugin.go: Reclassify FailedPrecondition errors for direct agent-profile messages (agent_unset, agent_missing, agent_ineligible) with 412 guidance pointing to Notes settings; retain legacy utility-agent classifier branches (agent_disabled, agent_unconfigured_profile) with Utility Agents guidance
• ui/bundle.js: Update error-action mappings to match new codes; unset/missing/ineligible → Notes settings, disabled/unconfigured → Utility Agents settings
• README.md: Rewrite setup instructions for one-step agent-profile selection; remove custom Utility Agent creation as a required step for new installs
• CHANGELOG.md: Document the 0.5.0 changes
• Tests: Go table-driven classifier tests, Node UI enhanceErrorAction tests — all pass

Motivation

The platform now supports direct agent profile selection (agent_profile with format agent-profile), letting operators pick a profile in Notes settings without creating a separate Utility Agent first. This reduces setup from two steps to one.

Testing

make test    # go test ./server/... (pass) + node --test ui/**/*.test.mjs (98/98 pass)
make vet     # go vet ./server/... (pass)
make package-host  # builds kandev-plugin-notes-0.5.0.tar.gz (SHA-256: 48db62de46...)

• Legacy utility-agent error codes (agent_disabled, agent_unconfigured_profile) verified through table-driven tests
• Direct agent-profile error codes (agent_unset, agent_missing, agent_ineligible) verified through same harness
• Unrecognized FailedPrecondition remains an execution error (502, no code leak)

Breaking Changes

None. Legacy utility-agent configurations continue to receive the same guided error messages pointing to Settings > Utility Agents. New configurations use the direct agent-profile picker in Settings > Plugins > Notes.

Related Issues

N/A