feat: configurable vault location via --vault-dir flag and BDSTORAGE_VAULT env var

[diksha78dev]

Closes #45

Problem

The vault and state DB were hardcoded to $HOME/.imprint/, making it
impossible to use a separate drive, run multiple vaults, or use bdstorage
in CI where $HOME is ephemeral.

Solution

• Added --vault-dir <PATH> global CLI flag (parsed before subcommand dispatch)
• Added BDSTORAGE_VAULT env var as fallback (handled via clap's env attribute)
• $HOME/.imprint/ remains the default when neither override is set
• Threaded the resolved vault dir through vault.rs and state.rs as a &Path parameter
• Updated the Data Locations & Storage section of README.md
• Added integration tests for both the CLI flag and env var

Priority Order

--vault-dir flag → BDSTORAGE_VAULT env var → $HOME/.imprint/ (default)

[Rakshat28]

@diksha78dev resolve merge conflicts and ensure your PR passes the CI checks. (Please see the project’s contribution guidelines before submitting a PR.)

[diksha78dev]

@Rakshat28 I have resolved merge conflicts
Kindly review my pr and let me know if there any changes required.
Thanks.

[Rakshat28]

Please fix the failing CI tests @diksha78dev

[Rakshat28]

still failing CI tests, please read contribution guidelines of the project before submitting PR

[Rakshat28]

Hey @diksha78dev, thanks for the PR!

Before we merge this, can you please record a short video showing the new --vault-dir flag and the BDSTORAGE_VAULT env var working as expected and upload it? Just a quick run-through of the order of priority will do.

Please add it and let me know when you do so I can review and merge.

[diksha78dev]

@Rakshat28 Here is the requested video demonstrating the --vault-dir flag and BDSTORAGE_VAULT environment variable priority order. Let me know if everything looks good to merge

Screen.Recording.2026-05-18.162140.mp4