Fix unarchive issue reopening and archive-done idempotency

[Copilot]

Two bugs when moving tasks back from archived/done state: (1) unarchived Issues remained closed on GitHub despite project board status updating, and (2) --archive-done required a follow-up run without the flag to stabilize.

Unarchive reopens closed Issues

• Added reopen_issue() GraphQL mutation and get_item() single-item fetch to GitHubProjectClient
• Unarchive path in execute_sync() now immediately applies task fields (status, etc.) and reopens the underlying Issue

# Before: only unarchived on the board, Issue stayed closed
client.unarchive_item(task.board_item_id)

# After: unarchive, apply fields, and reopen
client.unarchive_item(task.board_item_id)
_apply_task_fields(client, task.board_item_id, task, fields)
bi = client.get_item(task.board_item_id)
if bi and bi.content_type == "Issue" and bi.content_id:
    client.reopen_issue(bi.content_id)

--archive-done idempotency

• Moved remove_done_tasks() from post-sync to pre-sync so the file is cleaned before the sync plan is built
• Second run now finds no Done tasks in file, no-ops correctly instead of requiring a follow-up run

Tests

• 5 new tests covering unarchive+reopen (Issue, DraftIssue, failure tolerance) and archive-done idempotency (repeated runs, empty Done section)

TASKS.md

• Updated to reflect both tasks moved to Done

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[jack-champagne]

TASKS.md Sync Preview (dry run)

Action	Count
Create	0
Update	2
Archive	0
Unchanged	1

This is a preview. Changes will be applied when merged to main.