feat: Linear tracker integration

[erishforG]

Vision (v1.0: Ticket lifecycle complete)

Add Linear as a fourth tracker alongside Jira, GitHub Issues, and GitLab.

Proposal

[tracker]
provider = \"linear\"

[tracker.linear]
team = \"ENG\"
# Auth: PARSEC_LINEAR_TOKEN or LINEAR_API_KEY

Supported features

• parsec start — fetch Linear issue title
• parsec ticket — view Linear issue details
• parsec board — display Linear team board
• parsec create — create Linear issues
• Auto-transition (auto_transition config)

Tracker support comparison

Tool	Trackers
parsec	Jira, GitHub Issues, GitLab → + Linear
Graphite	Linear (web UI only)
Others	None

Milestone

v1.0