feat(agent): Linear adapter for project/milestone matching

[JSONbored]

Parent epic: #3180

Goal

Implement a LinearAdapter against the ProjectTrackerAdapter interface (introduced by #3183), so repos that track work in Linear instead of (or in addition to) GitHub Projects/Milestones get the same matching/suggestion/auto-apply behavior.

Requirements

• New autoProjectMilestoneMatchBackend: "github" | "linear" setting (default "github").
• Auth: Linear personal API key, sent as raw Authorization: <key> header (no Bearer prefix) — simplest single-workspace model, no OAuth needed for a self-host/single-org integration. Store as a per-repo or per-org secret, never committed.
• List open projects via Linear's GraphQL projects(filter: { status: { type: { neq: "completed" } } }).
• Prefer reading Linear's own native GitHub auto-link result where the integration is already installed on that workspace (Issue.attachments/GitHub-link metadata) over re-deriving a match, since Linear already auto-links PRs via its branch-name (<team-key>-<number>-<slug>) and magic-word conventions (linear.app/integrations/github) — only fall back to gittensory's own tokenize/termOverlap matching when Linear's integration isn't installed on that workspace.
• Issue.project / Project.issues for the project link; Issue.projectMilestone for milestone-level grouping if the repo wants milestone-equivalent granularity.

Deliverables

• LinearAdapter implementing ProjectTrackerAdapter.
• Backend-selection config + secret storage.
• Tests: Linear-native-link-present path, fallback-matching path, API-error best-effort path.

Expected outcome

A repo can point auto-project/milestone matching at Linear instead of GitHub Projects without any change to the matching/suggestion/auto-apply orchestration — confirms the adapter boundary was real, not just theoretical.

References

• https://linear.app/developers/graphql
• https://linear.app/integrations/github
• https://github.com/linear/linear/blob/master/packages/sdk/src/schema.graphql

[linear-code]

JSO-2616