Enable env-backed config writeback and bootstrap missing config file

[CJackHwang]

Motivation

• Allow configurations provided via the DS2API_CONFIG_JSON env var to be persisted to disk when desired and to bootstrap a missing config file so runtime changes are not lost, while still preserving special-case behavior for Vercel.

Description

• Add envWritebackEnabled() to recognize DS2API_ENV_WRITEBACK truthy values and update imports to include fmt and path/filepath.
• Change loadConfig() to, when DS2API_CONFIG_JSON is present and writeback is enabled (and not Vercel), prefer an existing file-backed config if present and attempt to write a bootstrap file when the config path does not exist.
• Allow Store.Save() and store.saveLocked() to persist an env-backed store when writeback is enabled by calling writeConfigBytes() and clear s.fromEnv after a successful write, while retaining the previous skip behavior for Vercel or when writeback is disabled.
• Add helpers writeConfigFile() and writeConfigBytes() to marshal configs, create parent directories with os.MkdirAll, and write files safely.
• Add unit test TestEnvBackedStoreWritebackBootstrapsMissingConfigFile to verify bootstrapping from env JSON and subsequent persistence and reload semantics.

Testing

• Ran the package unit tests for the config package with go test ./internal/config -run TestEnvBackedStore*, which executed the new TestEnvBackedStoreWritebackBootstrapsMissingConfigFile and related tests, and they passed.
• Ran wider checks with go test ./... to ensure no regressions in other packages, and all tests passed.

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Codex Task

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Project	Deployment	Actions	Updated (UTC)
ds2api	Ready	Preview, Comment	Mar 30, 2026 3:40pm

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