Implement CredentialStore for Windows (DPAPI) and Linux (libsecret)

## Context
`CredentialStore` (`src/Fallout.Build/Utilities/CredentialStore.cs`) is the OS-keychain backing for step 5 of the ADR-0002 secret resolution chain — but it's **macOS-only today**. `SavePassword` / `TryGetPassword` short-circuit on `PlatformFamily.OSX`; Windows and Linux users fall back to prompt-every-run.

This gets painful as the v12 plugin SDK lands — Windows plugin authors are the first cohort to routinely hit `dotnet fallout :secrets`, and prompt-fallback becomes a day-one stumbling block.

## Why now (v11, not v12)
Flagged in ADR-0002 as a v12 entry, but pulling into v11 because:
- v11 is where the plugin foundation lands. Better to have Windows credential storage working *before* the SDK ships.
- Both impls are well-trodden — DPAPI on Windows is a one-file `ProtectedData` wrapper; libsecret on Linux is a P/Invoke or `secret-tool` shell wrap.
- Unlike most ADR-0002 follow-ups, this has a concrete, scoped surface — no design ambiguity.

If load suggests it's too big for v11, move to v12 (the original ADR placement).

## Scope
- **Windows:** `SavePassword` / `TryGetPassword` for `PlatformFamily.Windows` via `System.Security.Cryptography.ProtectedData` (DPAPI, user scope). Storage: a small file under `%LOCALAPPDATA%\Fallout\credentials\` or registry under `HKCU\Software\Fallout\Credentials\` — pick one and document.
- **Linux:** same surface via libsecret. P/Invoke libsecret-1, shell-out to `secret-tool` (simpler, needs a binary), or a freedesktop.org Secret Service D-Bus call. Pick the simplest that handles the GNOME Keyring / KWallet duopoly.
- **Tests:** at minimum round-trip save+retrieve+delete on each platform behind `[OSPlatformFact]` (or whatever per-platform xUnit gating already exists).
- **Docs:** `docs/secrets.md` updated to drop the "macOS-only" caveat.

## Out of scope
- Cross-platform key import/export. Each platform stores under its native primitive; values don't roam.
- Headless Linux fallback. If neither GNOME Keyring nor KWallet is present (e.g. CI runners without a session), the prompt fallback stays as last resort.
- `keyring`-style consolidation libraries — keep the surface minimal and stdlib-first.

## Acceptance
- [ ] `CredentialStore.SavePassword("test", "value")` round-trips on Windows (DPAPI) and Linux (Secret Service or libsecret).
- [ ] `CredentialStore.TryGetPassword("missing", out _)` returns `false` without throwing on both.
- [ ] macOS path unchanged.
- [ ] ADR-0002's "Open Questions" entry struck through with a link back here, then to the implementing PR.

## Related
- ADR-0002 — Cross-provider auth and secret conventions (`docs/adr/0002-cross-provider-auth-and-secret-conventions.md`, "Open Questions"). Lands via #177.
- #167 — ADR-0001/0002 review (where ADR-0002 flips to Accepted).
- v12 milestone — original ADR-0002 placement; pulled forward to v11.