R1: Security hardening for first public release

[jmcte]

Goal

Close the public-exposure gap before flipping the repo public and tagging v0.1.0. The custom secret scanner, SQLite storage permissions, and disclosure docs are all sized for a private project today; once the repo is public, contributors and operators need clearer guarantees.

This is the first of six release milestones (R1–R6) leading to v0.1.0. See the roadmap in #75 follow-ups; companion epics: R2, R3, R4, R5, R6 (filed separately).

Sub-tasks (existing issues, must close before R1 is done)

• Harden SQLite metadata file permissions #79 — Harden SQLite metadata file permissions (0600 on newly-created DBs)
• Add local secret-scan mode for untracked files #80 — Add local secret-scan mode for untracked files
• Document secret scanner limits and add MailPlus leak patterns #81 — Document secret scanner limits and add MailPlus leak patterns

Additional acceptance criteria

• SECURITY.md reviewed for public-disclosure clarity: a non-internal reporter can find a contact channel and an SLA expectation.
• docs/privacy-redaction-boundaries.md linked from README.md and CONTRIBUTING.md so newcomers see it before submitting fixtures.
• fixtures/ audited once more for synthetic-only content; no real domains, no real names. Add a one-line README in fixtures/ confirming the policy and how to extend safely.
• scripts/check-detect-secrets.sh documented as fast guardrail (per Document secret scanner limits and add MailPlus leak patterns #81), and the --all-local mode from Add local secret-scan mode for untracked files #80 surfaced in CONTRIBUTING.md.
• No regression in fast-CI runtime: scanner additions stay under the existing time budget.

Out of scope

• Full DLP / pattern coverage parity with commercial scanners — explicitly deferred.
• Live MailPlus credential handling — covered by the live adapter milestone, not here.

Definition of done

• All four checkboxes above and all three sub-issues are closed.
• bash scripts/ci/run-fast-checks.sh still green.
• mpi doctor still green on a clean clone.