feat: Implement skill to add evm networks on extension repo

[GeorgeGkas]

Description

Moves the hand-authored metamask-extension Cursor skill at
.cursor/skills/mms-add-evm-swaps-bridge-network/SKILL.md into this shared
skills repo as a metamask-extension overlay of add-evm-network, and
finishes an in-progress rename of add-evm-network-mobile → add-evm-network
so the skill now covers both Extension and Mobile.

Previously, add-evm-network-mobile only had a Mobile overlay. A
repos/metamask-extension.md overlay had already been added under the
renamed add-evm-network/ directory, but two things were left unfinished:

• the copied overlay still contained @docs/add-evm-swaps-bridge-network.md,
  a Cursor-only @file mention that only resolves inside the
  metamask-extension workspace — meaningless once installed into
  .claude/skills/, .cursor/rules/, or .agents/skills/ in any consumer
  repo;
• the base skill.md still had name: add-evm-network-mobile and a
  Mobile-only description, even though a metamask-extension.md overlay
  now sat next to metamask-mobile.md. This meant the Extension overlay was
  installing under the wrong name (mms-add-evm-network-mobile) instead of
  mms-add-evm-network.

This PR:

• Fixes the reference in repos/metamask-extension.md to a plain
  `docs/add-evm-swaps-bridge-network.md` repo-relative path, matching
  the convention already used by the sibling add-non-evm-network skill.
• Renames skill.md's name to add-evm-network and broadens its
  description to cover both MetaMask Extension and MetaMask Mobile.
• Updates the parent: frontmatter in both repos/metamask-extension.md and
  repos/metamask-mobile.md to match.
• In the metamask-extension repo (out of scope for this repo's diff, but
  done as part of finishing the move): removed the now-redundant
  .cursor/skills/mms-add-evm-swaps-bridge-network/, regenerated the managed
  skill via tools/install --repo metamask-extension --domain swaps, and
  updated AGENTS.md's "EVM Swaps/Bridge Agent Entrypoints" section to point
  at the synced .claude/skills/, .cursor/rules/, and .agents/skills/
  outputs instead of the old local file (mirroring the "Non-EVM" section's
  pattern).

Type of Change

• New skill
• Skill improvement/update
• Bug fix
• Documentation update
• Other (please describe):

Skill Details (if adding a new skill)

Provider Name: MetaMask

Skill Name: add-evm-network (formerly add-evm-network-mobile)

Brief Description: Add a new EVM network to the unified Swaps/Bridge flow
in MetaMask Extension or MetaMask Mobile — bridge chain allowlist, default
destination token, stablecoin slippage, and bridgeConfigV2 rollout.

Checklist

• I have read the CONTRIBUTING.md guidelines
• My skill follows the SKILL_TEMPLATE.md format
• I have tested this skill with an AI agent
• My skill does not contain any secrets, private keys, or sensitive data
• I have added appropriate documentation
• My changes don't break existing skills

Testing

• tools/install --repo metamask-extension --target <tmp> --domain swaps --dry-run /
  without --dry-run, then inspected the merged
  .claude/skills/mms-add-evm-network/SKILL.md — confirms the overlay now
  installs under mms-add-evm-network (previously mms-add-evm-network-mobile)
  with the corrected docs/add-evm-swaps-bridge-network.md reference.
• tools/install --repo metamask-mobile --target <tmp> --domain swaps —
  confirms the Mobile overlay still installs correctly under the renamed
  skill.
• node --test test/*.test.mjs — no new failures (two pre-existing failures
  are sandbox/EPERM filesystem issues unrelated to this change).
• node .github/scripts/lint-skill-entry.mjs (yarn audit:skills) —
  48 skill(s) checked, 0 error(s); only pre-existing repo-wide warnings.
• Installed into a real metamask-extension checkout
  (tools/install --repo metamask-extension --domain swaps), removed the
  stale local .cursor/skills/mms-add-evm-swaps-bridge-network/, and
  verified the regenerated .cursor/rules/mms-add-evm-network/RULE.md,
  .claude/skills/mms-add-evm-network/SKILL.md, and
  .agents/skills/mms-add-evm-network/SKILL.md render the same corrected
  content.

Additional Context

N/A