feat: keep the vaults in one directory with DECANT_VAULTS

[GeisonJr]

What

• DECANT_VAULTS pins the vaults to one absolute directory, instead of vaults/ under wherever the command ran.
  • A relative value is refused; a leading ~ is expanded even when the shell left it quoted.
  • Only the last level is created — a missing parent stops the run instead of building a tree where nobody will look.
  • A run that creates the directory and then fails removes it again while it is still empty. This also fixes the empty vaults/ a failed first run used to leave behind.
• Vaults by name. view, analyze and credits take a vault's name as well as its path, looking the name up in the vaults directory. A path that does not exist is still an error, not retried as a name.
• The analysis agent runs inside the vault. Every agent is started with permission to write only in its working directory, which was wherever the command ran, so a vault outside it was out of reach. The prompt now names the vault's files relative to it.

Why

From a clone, bun run decant always runs in the repository root, so the vaults live inside the repository — ignored by git, and so within reach of git clean -fdX. Installed globally, they scatter across whatever directory the command ran in.

Testing

• bun run typecheck
• Manual runs against scratch directories:
  • relative DECANT_VAULTS → error, exit 1, nothing created
  • quoted ~/… with a missing parent → expanded to the home directory, run stops, nothing created in the working or home directory
  • new directory under an existing parent, failed run → created, then removed
  • default ./vaults, failed run → created, then removed
  • existing directory with content, failed run → left intact
  • a short generated video → the vault lands in DECANT_VAULTS, no .staging left behind
  • view, credits and analyze by name (with and without the variable), by path, with an unknown name and with a missing path
  • analyze with a stub claude on PATH that prints its working directory and writes NOTES.md → runs inside the vault, even when the vault is outside the current directory
• Not tested: a real analysis with Claude Code, Codex or Gemini running inside the vault.