Adds plugins/web/perplexity — a keyed-only WebSearchProvider over httpx:
- search: POST https://api.perplexity.ai/search (documented Search API),
search_context_size=low so `snippet` stays description-sized;
results[].snippet -> description, max_results capped at the API's 20.
- extract: POST /sdk/content/snippets — the query-relevant page-excerpt
route behind `pplx content snippets` (the CLI's `content fetch` is
deprecated upstream). web_extract has no query, so the URLs' path words
serve as the relevance query; per-URL `error` entries survive a 200.
- Wired into the same touchpoints as the other keyed vendors: legacy
backend set + credential ladder + availability probe (web_tools),
registry preference walk, OPTIONAL_ENV_VARS, `hermes config`/status/
dump key lists, nous_subscription direct-credential detection, setup
summary, test conftests, docs.
Not a keyless-ring member (Perplexity has no anonymous tier). Related
closed PRs #9192 / #23981 / #45225 predate the plugin ABC.
