Add timestamp-based package filtering (exclude-newer / exclude-older)

[Copilot]

Adds support for filtering mirrored packages by build timestamp, enabling use cases like "mirror only packages built after 2023".

Changes

• PackageFilter replaces MatchSpecWrapper — extends MatchSpec with optional exclude-newer / exclude-older timestamp bounds. Both plain-string and object YAML forms are supported and can be freely mixed.
• parse_datetime — accepts RFC 3339 (2025-01-01T00:00:00Z), date-only (2025-01-01 → midnight UTC), and relative durations (90d = 90 days ago).
• MirrorMode variants updated to carry Vec<PackageFilter> instead of Vec<MatchSpec>.
• get_packages_to_mirror — delegates to PackageFilter::matches(). Also fixes a pre-existing logic bug in IncludeExclude where packages matching an include override were still being excluded when an exclude list was present.
• Packages with no timestamp field always pass the bounds check (include-unknown).
• 13 unit tests added for datetime parsing, deserialization, and matching logic.

Example config

include:
  - "jupyter*[license=MIT]"             # unchanged plain-string form

  - matchspec: "python*"
    exclude-newer: "2025-10-01T00:00:00Z"
    exclude-older: "2023-01-01"

  - matchspec: "numpy*"
    exclude-older: "90d"                 # relative: only packages from last 90 days

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[github-actions]

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[pavelzw]

@copilot fix CI

[Copilot]

@copilot fix CI

Fixed in c18f9a9. The CI failures were:

1. Clippy collapsible_if — three nested if let blocks were collapsed into if let ... && ... chains.
2. rustfmt — import ordering in config.rs and lib.rs was reordered to match rustfmt's expectations.

[justacec]

Was all off the code for this PR just vibe coded?

Not used to seeing a bunch of copilot messages with no human interactions.

[pavelzw]

Yes I tried it out back then with semi success. Feel free to pick this thing up or start from scratch if you want :)