feat: archive closed tickets for large stores

[johnurbanik]

What changed

• adds tk archive [id...] and tk archive --reindex
• keeps archived Markdown tickets available to show, closed, ls, query, partial-ID lookup, and dependency commands through a compact metadata index
• restores archived tickets to the active directory when reopened
• fixes tk closed SIGPIPE failures on stores with more than 100 files

Why

Large repositories pay the cost of opening every closed ticket for routine graph reads. The Glass had 1,244 ticket files, including 946 closed tickets, causing multi-second Windows/MSYS command latency.

Impact

Closed history remains intact while ready, blocked, show, and graph commands scan only active Markdown files plus one compact index. Relationship edits require reopening an archived ticket to keep the index authoritative.

Validation

• archive feature: 7 scenarios, 55 steps passed
• closed listing regression: 3 scenarios, 21 steps passed
• Bash syntax checks passed for core and modified plugins
• large-store migration preserved ready, blocked, and show output exactly