chore: add agent skills configuration

[tobyhede]

Adds per-repo configuration for the engineering agent skills.

• AGENTS.md with an ## Agent skills section pointing at the docs below
• docs/agents/issue-tracker.md: issues live in Linear, team CIP, labelled envelopers; accessed via the Linear MCP tools
• docs/agents/triage-labels.md: default five triage roles (needs-triage, needs-info, ready-for-agent, ready-for-human, wontfix) as Linear labels
• docs/agents/domain.md: single-context layout (root CONTEXT.md + docs/adr/), created lazily

Docs only; no code changes.

[freshtonic]

Standards

No findings. The repository has no applicable documented coding standard for these files, and the docs introduce no relevant maintainability smell.

Spec

No merge-blocking findings. The PR adds the requested agent-skills configuration, remains docs-only, passes git diff --check, and both CI checks are green.

Two non-blocking documentation follow-ups:

• docs/agents/domain.md also documents a multi-context layout even though this repository is configured as single-context. Removing that extra section later would make the policy more direct.
• docs/agents/issue-tracker.md names the Claude-specific Linear connector even though AGENTS.md is agent-neutral. Provider-neutral “Linear MCP tools” wording would avoid misleading other agents.

Approved; neither documentation point needs to hold up this PR.