feat(config): add central server config library

[trillium]

Overview

Eliminates the need to duplicate server connection settings (host, port, user, mode, TLS) across 30+ per-project metadata.json files by introducing a central config library that can load machine-wide defaults from ~/.config/beads/server.json.

Changes

• Add LoadCentralConfig(), DefaultCentralConfigPath(), and ApplyCentralDefaults() to the configfile package
• Wire applyCentralConfigDefaults() into NewFromConfigWithOptions and NewFromConfigWithCLIOptions in internal/storage/dolt/open.go

Design decisions

• Only server connection fields are inherited from central config — per-project fields (database, data-dir, project-id) are never inherited
• Per-project values always override central defaults
• Missing central config is a silent no-op
• Central config path overridable via BEADS_CENTRAL_CONFIG env var

Test plan

Run go test ./internal/configfile/ -run TestLoadCentral -v and go test ./internal/configfile/ -run TestApplyCentral -v to validate the library, and go test ./internal/storage/dolt/ -run TestApplyCentralConfigDefaults -v to validate the integration wiring:

• Loading a valid central config file returns the expected fields
• Missing file returns (nil, nil) — no error, graceful skip
• Invalid JSON returns a parse error
• Empty project fields are filled from central defaults
• Non-zero project fields are preserved over central values
• Env vars (BEADS_DOLT_SERVER_HOST, etc.) override both project and central via getter methods
• Per-project-only fields (database, data-dir, project-id) are never inherited
• nil central config is a no-op
• DefaultCentralConfigPath() returns a path ending in beads/server.json
• applyCentralConfigDefaults loads and applies central config to per-project config
• Missing central config file is a no-op in the wiring path
• Project fields override central defaults through the wiring path

Commits

1. test(config): add specs for central server config library — Stub implementations + full test suite. Three tests fail (LoadCentralConfig_FileExists, LoadCentralConfig_InvalidJSON, ApplyCentralDefaults_FillsEmptyFields), defining expected behavior before implementation.
2. feat(config): implement central server config library — Fill in LoadCentralConfig (read + parse JSON, graceful skip on missing file) and ApplyCentralDefaults (merge server-only fields at zero values). All 9 tests pass.
3. test(config): add integration specs for central config wiring — Stub applyCentralConfigDefaults in open.go + integration tests. One test fails (WiresIntoOpenPath), proving the glue function doesn't yet load/apply the central config.
4. feat(config): wire central config into store open paths — Implement applyCentralConfigDefaults and call it in both NewFromConfigWithOptions and NewFromConfigWithCLIOptions. All integration tests pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 63.63636% with 16 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
internal/storage/dolt/open.go	30.76%	6 Missing and 3 partials ⚠️
internal/configfile/central_config.go	77.41%	4 Missing and 3 partials ⚠️

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