Bump sigil-mcp to 0.16.5

Gives the dotenv-migrated tree a version that unambiguously identifies it.

Until now two different trees answered to 0.16.4: the v0.16.4 tag points at 1a017593, which imports (sigil env), while master's tip 52d7e90e ("Migrate dotenv helpers out of sigil env") also declared version 0.16.4 and does not. A consumer resolving by tag and one resolving by package metadata got different code from the same version string.

That divergence had a real cost. The monorepo deliberately pinned 52d7e90e until the coordinated 0.18 tag, but a caret relock resolved 0.16.4 to the tagged 1a017593 and silently un-pinned it. Because the CLI loads (sigil cli mcp) at command registration, and post-rename (sigil env) resolves to the env authoring facade rather than the old dotenv helpers, that dragged authoring machinery into lock-only consumption paths and tripped the lock-only integration gate. Three successive wrong diagnoses were chased before anyone compared dependency SHAs.

v0.16.4 is left pointing where it always did rather than rewriting a published tag. Consumers wanting the dotenv-migrated tree take 0.16.5.