feat: support sysand add/remove PURL shorthand

[consideRatio]

Users can identify indexed packages as publisher/name, but
sysand add previously required the full
pkg:sysand/publisher/name IRI. The CLI also parsed the positional
add locator as an IRI before core add logic could apply add-specific
behavior.

Expand valid Sysand PURL shorthand in the add path so normalized
publisher/name input is stored as the canonical PURL. Reject shorthand
that would require normalization first. Leave other add resources to
existing usage validation. Keep the shorthand scoped to add by leaving
general project usage validation and remove parsing unchanged. Update
CLI parsing, docs, and Python binding coverage for the new add behavior.

[andrius-puksta-sensmetry]

I think sysand remove should also have this to preserve symmetry.

[consideRatio]

Hmmm this became more work than I have time to allocate to make deadlines. Do you have time to finalize/re-work it?

Decision required about core change vs CLI change:
a) We adjust add/remove in core, influencing all surfaces
b) We adjust add/remove in CLI only

My take is that this change isn't CLI specific, so it should be implemented in the core layer, but what we do matters less to me than we move forward.

If we would reprompt AI to implement this from scratch, I'd describe it as we want add/remove functionality in the core layer updated to:

• we ensure normalized publisher/name strings expands to pkg:sysand/publisher/name
• we error clearly on normalizable but not normalized publisher/name
• we otherwise don't influence any other behavior (CLI help for "if you meant to use a path" etc is retained)

[andrius-puksta-sensmetry]

Done everything to my liking. Now core do_add/do_remove don't do guessing, there are wrapper functions do_add_guess and do_remove_guess for this. CLI verifies/parses the PURL once.