feat: Add pgschema.toml configuration file support

[NFUChen]

Summary

Add pgschema.toml configuration file support, enabling users to run pgschema plan / pgschema apply without repeating CLI flags. Includes named environment overrides, multi-tenant schema loop, and CI workflow improvements.

Configuration file (pgschema.toml)

• New cmd/config package: TOML parsing with BurntSushi/toml, base + [env.*] merge logic using toml.MetaData.IsDefined() to correctly handle boolean zero-value overrides (e.g. auto-approve = false in env block overrides auto-approve = true at base level)
• --config and --env persistent flags on root command
• Priority chain: CLI flags > env vars > config env > config base > built-in defaults
• All three commands (plan, apply, dump) read config values as fallback when CLI flags are not explicitly set

Multi-tenant schema loop

• [schemas] config section with a SQL query field that discovers tenant schema names dynamically
• plan and apply iterate over all discovered schemas automatically
• dump is excluded — it produces a single template schema, looping would be redundant
• Multi-schema functions call core logic directly (GeneratePlan, ApplyMigration) to avoid infinite recursion through the dispatch check
• Schema discovery query runs in a read-only transaction (sql.TxOptions{ReadOnly: true}) to prevent accidental data modification

CI workflow improvements

• ci-test.yml: Split into unit (fast, go test -short) and integration (matrix across PG 14-18) jobs running in parallel
• release.yml: Converted serial PG version testing into parallel matrix strategy
• docker-latest.yml: Switched from Docker Hub to GitHub Container Registry (GHCR)

Changed files

File	Change
cmd/config/config.go	New config package: TOML parsing, env merge, schema discovery
cmd/config/config_test.go	Unit tests for config merge, boolean overrides, error cases
cmd/config_integration_test.go	Integration tests for config loading + read-only enforcement
cmd/root.go	--config, --env flags and loadConfig() in PersistentPreRun
cmd/plan/plan.go	Config fallback, multi-schema loop, removed MarkFlagRequired("file")
cmd/apply/apply.go	Config fallback, multi-schema loop
cmd/dump/dump.go	Config fallback only (no multi-schema)
README.md	Configuration file documentation section
.github/workflows/ci-test.yml	Matrix strategy for parallel PG version testing
.github/workflows/release.yml	Matrix strategy for parallel PG version testing
.github/workflows/docker-latest.yml	Switch to GHCR

Test plan

• go test -v ./cmd/config — unit tests for config parsing and merge logic
• go test -v ./cmd -run TestConfigLoading — integration tests for config file loading through root command
• go test -v ./cmd -run TestDiscoverSchemas_ReadOnlyEnforcement — read-only transaction enforcement (SELECT allowed, CREATE/DROP/INSERT rejected)
• go test -v ./... — full test suite to verify no regressions
• Manual test: create pgschema.toml with connection details, run pgschema plan without flags
• Manual test: create config with [env.dev] / [env.prod], verify --env switching
• Manual test: verify CLI flags override config values