docs: comprehensive update for server/storage/observability config st…

…ructure

Updated documentation to reflect the new configuration structure:

- server: Network and security (port, auth, tls, cors, studio)
- storage: Data persistence (tasks, sessions, memory, checkpoint)
- observability: Telemetry (tracing, metrics)

Fixed files:
- docs/guides/configuration.md: Complete Structure, Server Defaults sections
- docs/guides/deployment.md: Docker Compose, metrics, tracing examples
- docs/guides/memory.md: Memory index configuration (storage.memory)
- docs/guides/observability.md: All YAML examples fixed

All examples now use correct YAML structure with proper indentation.

fix: fail explicitly on partial auth configuration

Partial auth config (some fields but not all) now fails with a clear error
message listing which fields are missing. This prevents accidental insecure
deployments where users think auth is enabled but it silently disabled.

All three fields (auth-jwks-url, auth-issuer, auth-audience) are required
to enable auth, otherwise remove all auth flags completely.

fix(ci): remove stale UI generation from CI workflow

The embedded UI was removed from Hector. This removes:
- go:generate directive from pkg/server/doc.go
- Node.js setup steps from CI workflow
- 'go generate ./pkg/server' steps from CI workflow

Updates pkg/server doc to reflect the headless server architecture.

fix(ci): remove stale UI generation from CI workflow

The embedded UI was removed from Hector. This removes:
- go:generate directive from pkg/server/doc.go
- Node.js setup steps from CI workflow
- 'go generate ./pkg/server' steps from CI workflow

Updates pkg/server doc to reflect the headless server architecture.

feat: add dynamic version support via ldflags

- Add version.go with Version, GitCommit, BuildDate variables
- Update Makefile to inject VERSION from git describe --tags
- Update cmd/hector/main.go to prefer ldflags, fallback to module version
- Version now shows: Hector v0.9.0 (abc1234) built 2025-12-16T...