feat(deep-links): make PostHog Code a Linear custom coding tool target

[mateobelanger]

Problem

Closes #3576

Linear can open an issue directly in a coding tool (Settings → Code & reviews), and supports arbitrary tools launched from a URL template with {{issue.identifier}} / {{context}} variables. PostHog Code wasn't set up as one of those tools, so there was no one-click path from a Linear issue into a prefilled task.

Changes

Extends the existing posthog-code://new deep link so Linear can use it as a custom coding tool. Users paste this template into Linear (replacing OWNER/REPO):

posthog-code://new?repo=OWNER/REPO&source=linear&issue={{issue.identifier}}&prompt={{context}}

• Added optional source (attribution, shared across new/plan/issue routes) and issue (external identifier, e.g. ENG-123) params, plus prompt_length_chars telemetry to catch OS-level URL truncation. No new route/hostname — Linear passes full context in {{context}}, so there's nothing to fetch (unlike the GitHub issue route). prompt goes last in the template so truncation only ever clips prompt text.
• New Settings → Linear section with a copyable URL template (correct scheme per build) and setup steps; extracted the shared CopyableCommand component out of ClaudeCodeSettings.
• Documented both params and the Linear flow in docs/DEEP-LINKS.md.

The Linear agents integration (delegate an issue → agent works it → activity streamed back) is out of scope here — it lives on the PostHog Django backend.

How did you test this?

• Full pnpm typecheck, pnpm lint, and pnpm test pass locally. New unit cases cover source/issue parsing, rejection when neither prompt nor repo is present, unicode/newline round-trip, and analytics passthrough (new-task-link + newTaskLinkResolver).
• Live check against the running dev app: fired posthog-code-dev://new?repo=…&source=linear&issue=ENG-123&prompt=… and confirmed the main-process handler logged source: 'linear', hasIssue: true, correct repo and prompt length, and queued the pending link.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

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