feat(photon): support rich link previews

[huntsyea]

What changed

Adds full Photon rich-link support for Hermes:

• routes URL-only outbound Photon replies through the sidecar's /send-richlink endpoint, which wraps Spectrum richlink(url) so iMessage can render native preview cards
• falls back to plain /send text if richlink sending is unsupported or fails
• preserves inbound richlink URL text, and preserves title / summary metadata when Spectrum has already exposed those as plain fields
• coalesces iMessage .pluginPayloadAttachment preview artwork that arrives immediately after a URL/richlink so the agent receives one link message instead of a follow-up attachment prompt
• updates Photon docs and focused regression coverage

Why

Fixes #50336.

This also supersedes/extends #50337. That PR handles the inbound richlink placeholder bug; this branch includes compatible inbound handling and adds outbound richlink sending plus preview-art coalescing. Unlike #50337's original sidecar approach, this branch does not call lazy title() / summary() accessors in the inbound stream. Photon docs describe those as lazy metadata accessors, and webhook docs intentionally keep richlink deliveries URL-only to avoid adding URL fetch latency or attack surface to delivery.

Validation against Photon docs

Checked current Photon docs:

• https://photon.codes/docs/llms.txt lists Spectrum rich links and webhooks docs.
• https://photon.codes/docs/spectrum-ts/content/rich-links.md documents richlink(url) for outbound URL preview cards and lazy title(), summary(), cover() metadata accessors.
• https://photon.codes/docs/webhooks/overview.md says webhook richlink deliveries ship the url only and omit resolved title/summary/cover metadata.

Implementation follows that split: outbound uses richlink(url); inbound preserves URL and any already-materialized title/summary fields without resolving lazy accessors itself.

Tests

• uv run --with pytest==9.0.2 --with pytest-asyncio==1.3.0 python -m pytest tests/plugins/platforms/photon -q — 133 passed
• node --check plugins/platforms/photon/sidecar/index.mjs
• git diff --check

I also ran two reviews. One implementation review found no blocking issues.

One docs/API review flagged lazy inbound metadata resolution as risky; this was updated to avoid resolving title() / summary() in the sidecar and only preserve plain fields when present.

[teknium1]

Thanks for extending the Photon sidecar with an outbound rich-link path and preserving the v8-compatible inbound behavior.

Problems

• plugins/platforms/photon/adapter.py:1529 removes current main's return result after exhausted retryable attempts (plugins/platforms/photon/adapter.py:1450-1455). The new fallthrough performs another plain-text _sidecar_send() for every exhausted retryable send, including ordinary text. A timeout can occur after space.send() accepted the earlier message, so that extra POST can duplicate a delivered response. The richlink-specific fallback is already handled by _sidecar_send() at PR lines 1565-1574.

Suggested changes

• Restore the exhausted-retry return for ordinary sends and add a regression test asserting that an exhausted retryable non-URL response makes no extra send.

Automated hermes-sweeper review.

[teknium1]

Please retain the existing return result for exhausted retryable failures. This fallthrough runs for ordinary text as well as URLs and makes another /send after the retry budget; an HTTP timeout can occur after space.send() accepted a prior attempt, so that extra POST can duplicate a delivered reply. Richlink-to-plain fallback is already handled inside _sidecar_send().

[teknium1]

Merged via #73614 — cherry-picked with authorship preserved. Rich links + preview-artwork coalescing shipped; the preview-suppression logic composed cleanly with the FFFC wait machinery on main.