feat: add .claude-plugin/marketplace.json for plugin marketplace discovery

[leggetter]

Summary

Adds .claude-plugin/marketplace.json to the repo root so this collection is discoverable by Claude Code's plugin marketplace system (/plugin marketplace add hookdeck/webhook-skills) and by third-party indexers like claudemarketplaces.com.

Metadata-only change. No skill content modified.

What it enables

• /plugin marketplace add hookdeck/webhook-skills registers this repo as a marketplace.
• /plugin install stripe-webhooks@hookdeck-webhook-skills (and 36 more) installs a single skill.
• Third-party indexers that crawl .claude-plugin/marketplace.json (e.g. claudemarketplaces.com) get 37 individually-listed plugins with their own descriptions, categories, keywords, license, and source paths.

Grouping decision: one plugin per skill (granular)

Considered two options:

• (a) Granular — one plugin entry per skill folder. What this PR ships. 37 plugin entries, one per skills/<name>/.
• (b) Bundled — one provider-webhooks plugin grouping the 33 provider skills, plus separate entries for webhook-handler-patterns, the two hookdeck-event-gateway* skills, and outpost. Rejected.

Why (a):

• Matches existing install UX. The skills/CLI ecosystem already exposes per-skill installs (npx skills add hookdeck/webhook-skills --skill stripe-webhooks). The marketplace catalog should mirror that, not force users into a different mental model.
• Avoids over-pull. A user who wants only stripe-webhooks shouldn't have to install 32 other unused provider skills to get it.
• Better discoverability in indexers. A catalog crawler gets 37 entries with their own descriptions, keywords, and homepages — more searchable than one big bundle entry that buries 33 sub-skills under a single description.
• The catalog-noise concern (37 vs 4 entries) is real but doesn't outweigh the install ergonomics, and the per-skill keywords make the catalog filterable.

anthropics/skills' marketplace uses three bundled entries (document-skills, example-skills, claude-api), but those bundles are tightly themed — webhook providers are independent and users typically only want one or two at a time, so the bundling pattern doesn't transfer.

Schema choices

• name: hookdeck-webhook-skills — not a reserved name; this is what users type after @ when installing.
• owner: Hookdeck, contact phil@hookdeck.com.
• metadata.description / metadata.version: catalog-level description and 0.1.0.
• Each plugin entry uses source: "./" + strict: false + an explicit skills: ["./skills/<name>"], matching the pattern in anthropics/skills. The skill folders don't carry their own plugin.json today, so strict: false lets the marketplace entry be the complete plugin definition.
• category: integration for provider and infrastructure plugins (33 + 3 = 36 entries); development for webhook-handler-patterns (it's framework-agnostic coding patterns, not provider integration).
• license: MIT on every plugin, matching each SKILL.md frontmatter and repo LICENSE.
• author: Hookdeck.
• repository: this repo. homepage: the skill's GitHub directory URL on main.
• keywords: per-plugin (webhooks + provider + category-specific terms) for indexer searchability.
• No version on individual plugins: per the docs, omitting version makes the git commit SHA the version-resolution unit, so users get updates automatically when the repo updates. Easy to add explicit versions later if release cadence diverges per-plugin.

Mechanical details

• File generated by a small Python script (asserted every plugin's skills[0] path exists on disk before writing) so the 37 entries stay consistent. Script is not committed — the JSON output is the artifact.
• Coverage check: ls skills/ returns 37 folders; this manifest has 37 plugin entries; each entry's skills path resolves to a real folder.

Test plan

• cat .claude-plugin/marketplace.json | python3 -m json.tool > /dev/null — valid JSON
• python3 -c "import json; d=json.load(open('.claude-plugin/marketplace.json')); print(len(d['plugins']))" — reports 37
• Locally: /plugin marketplace add ./ from the repo root, then /plugin install stripe-webhooks@hookdeck-webhook-skills — verify the skill installs and /stripe-webhooks:* commands work.
• After merge: /plugin marketplace add hookdeck/webhook-skills from any fresh Claude Code session, install one provider skill, confirm the SKILL.md surfaces and references load.

https://claude.ai/code/session_01NNTgQRJss1V7gyzzJ9rjnB

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