chore: repair pnpm lockfile and pin prettier 3.9.6

[Swamp-Ig]

Summary

• Repair pnpm-lock.yaml by removing the duplicate @vuepress/helper@2.0.0-rc.122(…) snapshot key so pnpm install --frozen-lockfile works again (fixes pnpm-lock.yaml has duplicate key; --frozen-lockfile fails #2036).
• Bump prettier to ^3.9.6 / lock 3.9.6 — matching what CI was already resolving under the old ^3.8.1 caret while the broken lockfile was ignored. No source reformat needed; current tree already matches 3.9.x.
• Switch CI / documentation / publish installs to pnpm install --frozen-lockfile.
• Pin format-docs creyD/prettier_action to prettier_version: 3.9.6 so docs formatting stops floating on latest.

Stacking note

This is a base piece of upcoming entity-availability work (paired contrib + companion PRs, not opened yet). It pairs with the version-handshake split-off in #2039.

#2040 and #2039 are independent in substance (lockfile/CI vs handshake) and can be merged in either order. If they land out of the order a branch currently assumes, the remaining open PR(s) just need a clean rebase onto main (no conflict expected — disjoint files).

Fine to skip these intermediates and merge the eventual availability PR instead: that branch will carry both of these commits (plus the availability work), so merging availability alone is enough if preferred.

Test plan

• pnpm install --frozen-lockfile succeeds
• pnpm run lint clean
• pnpm test green (515 passed)
• CI matrix green on this PR

[Copilot]

Pull request overview

This PR restores deterministic dependency installs by repairing a broken pnpm-lock.yaml (duplicate snapshot key) and then tightening the repo’s tooling/CI to rely on the lockfile via pnpm install --frozen-lockfile. It also updates and consistently pins Prettier to 3.9.6 across dependencies and docs-formatting automation.

Changes:

• Fix pnpm-lock.yaml validity by removing the duplicated @vuepress/helper@2.0.0-rc.122(...) snapshot mapping key.
• Bump prettier to ^3.9.6 (and lockfile to 3.9.6) so dev tooling resolves consistently.
• Update GitHub Actions workflows to use pnpm install --frozen-lockfile, and pin the docs formatting action to Prettier 3.9.6.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pnpm-lock.yaml	Removes the duplicated snapshot entry and updates Prettier-related lock entries to 3.9.6, enabling frozen-lockfile installs.
package.json	Updates the dev dependency range for prettier to ^3.9.6.
.github/workflows/ci.yml	Switches CI installs to pnpm install --frozen-lockfile to catch lockfile drift/breakage.
.github/workflows/documentation.yml	Uses frozen-lockfile install for docs builds to ensure reproducible dependency resolution.
.github/workflows/publish.yml	Uses frozen-lockfile install during publish workflow to ensure published builds match the committed lock.
.github/workflows/format-docs.yml	Pins creyD/prettier_action to Prettier 3.9.6 to avoid formatting drift from latest.

Files not reviewed (1)

• pnpm-lock.yaml: Generated file