feat: add Linear mod

[overlord-letta]

Summary

Adds a publishable Linear mod with:

• batched search, issue reads, creates, updates, comments, and relations
• dry-run previews on every write tool
• best-effort expected-state guards for updates and comments
• a /linear command with an optional active-work panel

Why

Linear work often requires repeated one-ticket calls and approvals. The new primitives batch related operations while preserving per-item failures and explicit approval on every write surface.

Safety

• invokes the Linear CLI with execFile and structured GraphQL variables; no shell interpolation
• passes the CLI an operational environment allowlist rather than agent/cloud secrets
• delegates authentication to the Linear CLI system-keyring flow
• rejects ambiguous assignee matches and validates relation endpoints during dry runs
• discovers single-team workspaces automatically; multi-team setups opt in with LINEAR_TEAM_KEY

expected guards are intentionally documented as non-atomic: they re-read each issue immediately before its mutation but cannot provide compare-and-swap semantics through Linear.

Validation

• bun test ./tests — 22 tests, 89 assertions
• TypeScript no-emit check across source and tests
• npm run validate
• npm pack ./packages/linear --dry-run --json — expected 13-file package surface only
• isolated letta install + letta mods list
• loaded through the Letta mod engine: 1 command and 6 tools, no diagnostics
• live smoke: all four write tools returned planned dry-run results and a follow-up read confirmed no issue change
• confidentiality scan of the exact public diff and package tarball found no secrets, private tickets, workspace data, local paths, or internal links