feat(maniphest): Add assignee to task display and history

[holmboe]

Summary

Add assignee information to phabfive maniphest show output.

• Display current Assignee in regular task view (shows (none) if unassigned)
• Display assignee change history when using --show-history flag
• Resolve user PHIDs to usernames for readable output

Before/After

Regular task view

Before:

- Link: http://phorge.domain.tld/T47
  Task:
    Name: Set up production code repository
    Status: Open
    Priority: Normal
    Created: '2026-01-09T17:50:29'

After:

- Link: http://phorge.domain.tld/T47
  Task:
    Name: Set up production code repository
    Status: Open
    Priority: Normal
    Assignee: daniel.lindgren
    Created: '2026-01-09T17:50:29'

With --show-history

Before:

  History:
    Priority:
    - 2026-01-09T17:50:29 [↓] Triage → High
    Boards:
      Development:
      - 2026-01-09T18:24:09 [→] Backlog → In Progress

After:

  History:
    Assignee:
    - 2026-01-09T18:23:05 [•] (none) → admin
    - 2026-01-09T18:23:17 [•] admin → daniel.lindgren
    Priority:
    - 2026-01-09T17:50:29 [↓] Triage → High
    Boards:
      Development:
      - 2026-01-09T18:24:09 [→] Backlog → In Progress

Test plan

• phabfive maniphest show T47 displays Assignee field
• phabfive maniphest show T1 shows Assignee: (none) for unassigned tasks
• phabfive maniphest show T47 --show-history displays Assignee transitions
• uv run tox -e py310 passes

🤖 Generated with Claude Code

[Timpan4]

lgtm!