feat: allow any URI or a local name in agent placeholders

[tkuhn]

Closes #652.

The agent placeholder offered a typed term only when it started with http(s):// or matched the ORCID pattern, so a URI in one of the other allowed schemes (did:, ipfs:, …) or a name to be minted locally could not be entered at all — the dropdown just said "No results found".

Only the dropdown was in the way: TemplateContext already mints a plain name under the namespace of the nanopublication being published, and IriTextfieldItem.Validator already accepted one.

Changes

• Offer any URI in an allowed scheme (Utils.isUriValue), not just http(s) ones.
• Offer a plain name that the validator accepts (Utils.isPlainName) — last in the list, so that the known users or suggestions a term matches keep the top and a free-form option never preempts the highlighted first entry. This goes for guided choices as well: they suggest values but don't limit them, so a name for a resource that has no identifier yet belongs there too, minted under the prefix of the field or under the namespace of the nanopublication when the field has none.
• Restricted choices are left restricted: they don't offer such a name and don't mark one.
• Show a value that will be minted locally as local:john (mint locally), the note set apart from the term itself (span.mint-note, normal weight, italic — .select2-dropdown span.term is bold). The rule lives in TemplateContext.isToBeMinted, mirroring processValue, so a value under a prefix — minted under that prefix, which the field shows next to it — is not marked.
• Don't blow up on a value that is not a URI (createIRI throws), and render a value with no known name once instead of as "value (value)" — which also affected unknown http URIs before.
• Use the generic user icon rather than the logged-in user's picture for an agent that isn't a parseable IRI.
• Blank the href of a URI link built on a model when the value is not a URI, the way the String overload already does for local URIs.

Verification

AgentChoiceItemTest (9 tests) and GuidedChoiceItemTest (6 tests, one of them asserting that a restricted choice offers no made-up name) cover the suggestions, the display value and the minting; full suite green (1230 tests).

Driven in a browser on "Assigning a team member to a Space" — agent placeholder:

typed	dropdown
john	the matching John users, then local:john (mint locally)
did:plc:z72i7hdynmk6r22z27h6tvur	the DID as-is
john doe (with a space)	only the matching users — it can't be minted, so it isn't offered
0000-0002-1267-0234	the ORCID URI only, no near-duplicate

and its guided-choice space field:

typed	dropdown
nanodash	the Nanodash space, then local:nanodash (mint locally)
my-new-space	local:my-new-space (mint locally)
my new space	No results found

Selecting the local name and hitting Preview signs a nanopub whose team member is https://w3id.org/np/RAXwGpQe…/john.

🤖 Generated with Claude Code

[tkuhn]

@ashleycaselli See this.