Add read-only task evidence show command

[coreytshaffer]

Summary

Implements CR-098: Add Task Evidence Show Command.

This adds tc task show <task_id>, a read-only reviewer-facing CLI command that displays a task’s ledger-derived evidence chain without mutating state or verifying signatures.

Scope

• Adds tc task show <task_id>
• Displays stable task fields:
  • task id
  • title
  • status
  • accepted value
  • review decision
  • ledger event count
  • event timeline
• Handles missing tasks with bounded task_not_found output
• Explicitly states that signature verification is not checked by this command

Non-goals

• No ledger writes
• No identity registry loading
• No signature verification
• No signing, authority, routing, runtime, backend, or execution changes
• No historical ledger rewrites

Validation

• pytest tests/test_cr_098_task_show.py -q: passed
• Focused suite: passed
• Full regression before commit: 715 passed, 2 skipped
• tc doctor: OK
• Guardrail checks confirmed no signature/identity registry path is invoked by tc task show