Add opt-in pnpm links for source-owned outpost artifacts

[joshglogau]

Summary

Adds the smallest local integration layer requested for the source-owned outpost artifact packages consumed by @wireio/sdk-outpost.

• Extends the existing .pnpmfile.cjs hook with opt-in links to wire-ethereum/sdk-artifacts and wire-solana/sdk-artifacts.
• Activates those links only when WIRE_LINK_LOCAL_OUTPOST_ARTIFACTS=1 is set.
• Leaves normal and release installs on the exact registry versions declared by sdk-outpost.
• Refreshes the pnpm hook checksum without committing sibling link: paths.

This PR does not modify Bazel targets, the platform dependency graph, existing repository documentation, deployment manifests, RPC configuration, or capability state.

Dependency stack

This PR is stacked on #56. The local source packages are produced by:

• Wire-Network/wire-ethereum#176
• Wire-Network/wire-solana#414

The link is a development and cross-repository validation path only. Publication remains owned by each producer repository.

Draft state

The declared 0.1.0 producer packages are not public yet, so the portable lockfile cannot be completed from the registry. Keep this PR draft until both packages publish; then regenerate the lockfile normally and verify a frozen standalone install.

Verification

• With the flag unset, both dependencies remain at their declared registry versions.
• With the flag set, both dependencies resolve to the sibling source-package directories.
• sdk-outpost tests: 5 suites, 18 tests.
• sdk-outpost typecheck passes.
• Repository lint passes.
• Final diff is limited to .pnpmfile.cjs and its lockfile checksum.