feat(snippets): entrypoint-based render + sdk.yaml reduced to id

[kinyoklion]

Stacked on #404. Two coordinated schema changes that simplify the contract between this repo and its consumers.

1. Drop `ld-application.get-started-file(s)`; render walks entrypoints

The CLI no longer asks each sdk.yaml which file in the consumer to touch. The consumer (gonfalon, ld-docs-private) declares its own list of entrypoint directories, and the renderer walks them recursively, picking up any file that has the right extension (`.tsx` / `.jsx` / `.ts` / `.js` / `.mdx`) AND contains the `SDK_SNIPPET:RENDER:` sentinel.

# Before
snippets render --target=ld-application --out=/path/to/gonfalon

# After
snippets render --target=ld-application \
  --entrypoint=/path/to/gonfalon/static/ld/components/getStarted

`--entrypoint` is repeatable. Walks skip `node_modules`, `.git`, `.next`, `dist`, `build`, `.cache`, `coverage`, `out`, `.turbo` wholesale. Symlinks aren't followed. Overlapping entrypoints dedupe.

This decouples the consumer's directory layout from the snippet sources: gonfalon is free to reorganize `static/ld/components/getStarted/sdk/` however it wants without us shipping a release.

2. `sdk.yaml` reduces to `id:`

# Before — 11 fields, none of which the engine actually read except get-started-file*
id: python-server-sdk
sdk-meta-id: python
display-name: Python (server)
type: server-side
languages: [...]
package-managers: [...]
regions: [...]
hello-world-repo: launchdarkly/hello-python
ld-application:
  get-started-file: ...
docs:
  reference-page: /sdk/server-side/python

# After
id: python-server-sdk

The id matches the SDK's identifier in `launchdarkly/sdk-meta`'s API packages. Every other piece of per-SDK metadata (display name, language flavors, supported regions, hello-world repo URL, docs path) lives there as the authoritative source — this repo doesn't need to duplicate it. The `descriptor` struct and `loadDescriptor` function are deleted; nothing in the engine reads sdk.yaml anymore.

End-to-end check against gonfalon

Step	Result
Stash gonfalon's `static/ld/components/getStarted/sdk/` (26 files, 122 markers).	`/tmp/sdk-stashed-entrypoints`
`sed` every marker hash to `0`.	122 / 122 reset
`snippets render --entrypoint=...getStarted` (one --entrypoint, no `--sdks`, embedded FS).	24 of 26 files rewritten
`diff -r stash gonfalon-sdk`	exit 0 — byte-identical
`snippets verify --entrypoint=...getStarted`	`ok`

Companion changes

• feat: Composite action for synchronizing SDK snippets gh-actions#82 (the `sync-snippets` action) will need an `entrypoints` input added so the workflow can pass the directories through. Will update that PR once this lands.
• gonfalon's local sync workflow draft will be updated to declare `static/ld/components/getStarted` (and any future entrypoint dirs) as inputs.

Out of scope

• Dropping `ld-application.slot` from snippet frontmatter — separate PR per @rlamb's call.
• Removing other dead snippet-frontmatter fields (`kind`, `description`, `lang`, `sdk`) — pending a final decision on which to keep as required documentation.

---

Note

Medium Risk
Medium risk because it changes the render/verify CLI contract and the ld-application adapter’s file discovery logic (now walking directories and filtering by sentinel/extension), which could miss or newly include files in consumer repos if entrypoints are misconfigured.

Overview
Reworks ld-application rendering to be entrypoint-driven. The snippets render/verify commands now require one or more repeatable --entrypoint directories (replacing --out) and pass these through to the adapter.

Changes file discovery from sdk-declared targets to consumer-tree scanning. The ld-application adapter now walks each entrypoint recursively, skips common junk dirs (for example node_modules, .git, dist, build), ignores symlinks, filters to known extensions (.tsx/.jsx/.ts/.js/.mdx), and only processes files containing the SDK_SNIPPET:RENDER: sentinel; overlapping entrypoints are deduped.

Simplifies SDK metadata inputs. sdk.yaml is reduced to just id: across SDKs, the ld-application sdk.yaml descriptor loader is removed, and snippet frontmatter drops the ld-application hints struct (ld-application.slot). Tests and authoring docs are updated to reflect the new entrypoint workflow and discovery behavior.

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