feat(skills): add optional payments skills (Stripe Link, MPP, Projects) (#31343)

* feat(skills): add optional payments skills (Stripe Link, MPP, Projects)

Adds four optional skills under optional-skills/payments/ wrapping the
Stripe Link CLI, the Machine Payments Protocol (MPP) clients, and the
Stripe Projects CLI plugin. Plus a router skill (payments) that picks
between them based on user intent.

All four are gated [linux, macos] — Stripe's Link CLI does not yet
support Windows. The other CLIs (mppx, stripe projects) are
cross-platform on paper but the payments cluster moves as a unit until
Link CLI gains Windows support.

Skills:
- stripe-link-cli  - one-time virtual cards + Shared Payment Tokens
- mpp-agent        - HTTP 402 payments via mppx/Tempo/Privy/AgentCash
- stripe-projects  - provision SaaS services + credential sync
- payments         - router/index skill for the cluster

Hard invariants encoded in every skill:
- Card PANs/wallet keys never enter agent transcripts, logs, or memory
- Spend approvals are not self-bypassable (Link app / wallet UI / CLI prompt)
- Final totals confirmed with user before any --request-approval call
- Credential output files cleaned up after one-time use

Zero core touches. Skills install via:
  hermes skills install official/payments/<skill>

* chore(skills/payments): drop router skill — skills shouldn't depend on other skills

Removed optional-skills/payments/payments/ — the router skill that
existed to hand off between stripe-link-cli, mpp-agent, and
stripe-projects.

Per project convention: skills should be independently loadable; a
router is a footgun because (a) it assumes the loader will follow its
recommendation rather than just loading what the user asked for, and
(b) it duplicates the trigger logic that already lives in each
sub-skill's '## When to Use' section.

The three remaining skills declare their own triggers and routing
hints. The optional-skills catalog still groups them under '## payments',
which is the appropriate place for cluster-level discoverability.

Also drops 'payments' from each remaining skill's 'related_skills' list
and removes the corresponding entries from the docs catalog + sidebars.

* feat(skills/payments): fold in danhill-stripe review feedback

- mpp-agent: add link-cli as a client option (when Link is already set
  up, or the 402 challenge advertises method="stripe")
- stripe-link-cli: reframe Link account / payment method / approval app
  as first-run setup, not hard preconditions (CLI configures them on
  first run)
- regenerate the two affected optional-skills docs pages

website/docs/reference/optional-skills-catalog.md

@@ -162,6 +162,14 @@ hermes skills uninstall <skill-name>
 | [**unsloth**](/docs/user-guide/skills/optional/mlops/mlops-training-unsloth) | Unsloth: 2-5x faster LoRA/QLoRA fine-tuning, less VRAM. |
 | [**whisper**](/docs/user-guide/skills/optional/mlops/mlops-whisper) | OpenAI's general-purpose speech recognition model. Supports 99 languages, transcription, translation to English, and language identification. Six model sizes from tiny (39M params) to large (1550M params). Use for speech-to-text, podcast... |
 
+## payments
+
+| Skill | Description |
+|-------|-------------|
+| [**mpp-agent**](/docs/user-guide/skills/optional/payments/payments-mpp-agent) | Pay HTTP 402 APIs via Machine Payments Protocol (MPP). |
+| [**stripe-link-cli**](/docs/user-guide/skills/optional/payments/payments-stripe-link-cli) | Agent payments via Stripe Link — cards, SPT, approvals. |
+| [**stripe-projects**](/docs/user-guide/skills/optional/payments/payments-stripe-projects) | Provision SaaS services + sync creds via Stripe Projects. |
+
 ## productivity
 
 | Skill | Description |