M1.5: Add viper-based configuration

[bjcorder]

Labels: type:feature, area:cli, priority:high,
status:blocked, milestone:M1, phase:1

Summary

Integrate viper for configuration sourced from CLI flags, environment
variables, and a YAML config file, in that precedence order.

Acceptance criteria

• internal/config/config.go defines a Config struct with:
  • DBPath string (default ~/.forkguard/state.db,
    env FORKGUARD_DB_PATH)
  • CacheDir string (default ~/.forkguard/cache,
    env FORKGUARD_CACHE_DIR)
  • GitHubToken string (default empty, env GITHUB_TOKEN)
  • LogFormat string (default json, env FORKGUARD_LOG_FORMAT)
  • LogLevel string (default info, env FORKGUARD_LOG_LEVEL)
• Load(cmd *cobra.Command) (*Config, error) populates the config from
  flags, env vars, and a YAML file (in that precedence)
• Default config file location is ~/.forkguard/config.yaml
• Tilde expansion works correctly on all supported platforms
• Tests cover all three precedence sources

Implementation notes

Use github.com/spf13/viper v1.18+. Bind viper to the cobra root command
in internal/cli/root.go's PersistentPreRunE.

Dependencies

Depends on: #4

Verification

FORKGUARD_DB_PATH=/tmp/test.db ./forkguard --help
go test ./internal/config/...

---

[bjcorder]

Implemented in aadabd316f68.

Summary:

• Added viper-backed internal/config.Config loading defaults, YAML config, environment variables, and changed CLI flags in the required precedence.
• Added cross-platform leading-tilde expansion for DB and cache paths.
• Wired config loading into the cobra root command's PersistentPreRunE and exposed loaded config from command context.
• Added focused tests for defaults, YAML config, env overrides, flag precedence, and tilde validation.

Validation:

• FORKGUARD_DB_PATH=/tmp/test.db ./forkguard --help — passed
• go test ./internal/config/... — passed
• go test ./... — passed

Notes / follow-ups:

• None

[bjcorder]

Implemented in PR #39: #39

Implementation commits:

• aadabd316f68 — Add viper-based configuration

Validation:

• make generate — passed
• go build ./... — passed
• go vet ./... — passed
• go run github.com/golangci/golangci-lint/cmd/golangci-lint@v1.64.8 run ./... — passed
• go test ./... -race -coverprofile=coverage.out — passed
• ./forkguard init and ./forkguard ingest --upstream github.com/openssl/openssl with idempotency check — passed

Closure status:

• This issue is ready to close when the PR is merged.

Remaining notes:

• None