fix(security-posture): pin actions to SHAs, allowlist exact secrets, correct policy scope

Addresses all four points from review on PR #246.

WHY THIS MATTERS
A security-posture PR has to hold itself to the standard it introduces.
Each of the four fixes below closes a gap where the original version
either overstated its guarantees (policy text) or quietly weakened them
(scanner allowlists, mutable action refs).

1. SECURITY.MD SCOPE - THE HOSTED SERVICE EXISTS
The policy claimed "there is no hosted service", but the official Mike
website offers a cloud version at app.mikeoss.com. A researcher reading
the old text would conclude the hosted service was out of scope and
either not report or disclose publicly. The policy now explicitly takes
hosted-service reports through the same private channel (with the usual
safe-harbor expectations: own accounts only, non-destructive testing)
and narrows the exclusion to what it was always meant to cover:
independent third-party self-hosted deployments.

2. SUPPORTED VERSIONS - TAGS EXIST AND ARE UNSUPPORTED
The repository has tags v0.1.0 through v0.4.0, so "there are no tagged
releases" was simply wrong, and wrong in the dangerous direction: a user
pinned to v0.4.0 might assume they were covered. The policy now lists
the tags explicitly as unsupported historical snapshots - only the tip
of main receives security fixes.

3. WORKFLOW ACTION PINNING - TAGS ARE MUTABLE, SHAS ARE NOT
`uses: some/action@v4` re-resolves on every run: whoever controls that
tag controls code that executes inside this repo's CI, which is exactly
how the 2025 tj-actions/changed-files compromise spread. The risk is
sharpest in the Scorecard job, which holds id-token: write (OIDC
signing) and security-events: write. Every `uses:` now references a
full commit SHA - the only immutable ref format GitHub offers - with
the release version kept in a trailing comment, which is the convention
Dependabot parses to keep proposing pinned updates:
    actions/checkout@11d5960a...       # v4.4.0
    github/codeql-action/*@a2983b8b... # v3.37.4
    ossf/scorecard-action@4eaacf05...  # v2.4.3

4. GITLEAKS - ALLOWLIST EXACT SECRETS, NEVER PATHS
The old config allowlisted entire files ("this file contains a fake
secret, ignore it"). gitleaks applies a global allowlist's `paths` as a
whole-file skip, so a REAL credential later committed to any of those
files would sail through the scan - the allowlist grew a permanent
blind spot with every entry. Reproduced before fixing: planting a
high-entropy secret in an allowlisted fixture file and scanning full
history reported zero leaks under the old config.

The new config allowlists exact secret VALUES instead - one anchored
regex per hand-verified fake (the safeError test fixtures, the add-in
e2e mock token, and Supabase's published local-dev demo JWTs, which are
public by construction). A real secret can never equal a known fake, so
the blind spot is gone; the same planted-secret experiment now fails
the scan, and the full history of both this branch and current main
still passes clean. This also surfaced two findings the old path list
missed entirely (the demo JWTs in .env.example / docker-compose.yml
history) - they are now consciously allowlisted by value rather than
accidentally green.

VERIFICATION
- All three workflows parse as valid YAML.
- gitleaks 8.30.1 (checksum-verified binary): full-history scan clean
  on this branch and on current origin/main with the new config.
- Planted-secret experiment: old config 0 leaks (blind), new config 1
  leak (caught) on an identical history.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>