feat(output): add --fields to limit JSON listings to named keys

[ryanlewis]

Adds --fields to limit -j output to the named keys of each row, like gh --json title,url. Without it, a 94-task anytime listing is 90,150 bytes, and agents that cap output with head -c 6000 see about 6% of it.

things anytime -a "Personal Projects" -j --fields uuid,title,projectTitle,deadline,tags

On a copy of a real database this prints 15,736 bytes instead of 90,150. On logbook it prints 129,822 bytes instead of 778,483 (with uuid,title,deadline).

Where it works

• Every task listing: today, inbox, upcoming, anytime, someday, repeating, logbook, trash, deadlines, and list with filters.
• search, which shares the listing code path.
• things projects.

things show does not take it and always prints the full record. areas and tags rows are short, so they were left out.

Decisions

• Names are the JSON keys exactly as printed, camelCase and case-sensitive, with no aliases. The todo's example project is rejected, and the error lists projectTitle among the valid names. Task listings take the task keys and projects takes the project keys. Both lists are read from the model's json tags, so a new field becomes selectable without a second list.
• Order follows the order the user gave.
• A requested key the full record would omit on a row (an empty omitempty value) is omitted here too. Values are cut from the marshalled record, not re-encoded, so asking for every key prints the full record byte for byte. Tests and a binary comparison against main on a copied database check this.
• Spaces around names are trimmed and repeats dropped; the first occurrence keeps its position.
• An unknown name fails before the database is opened, with a new error token unknown field carrying kind, unknown (the names not recognised) and valid (every accepted name).
• An empty list (--fields "" or ,) fails with the generic error token.
• --fields without --json fails with "needs --json". Plain output is unchanged.
• The projection lives in internal/output/fields.go and works on the marshalled rows generically.

Docs

• docs/content/commands.md: new section on choosing fields.
• docs/content/agents.md: the new error token.
• README.md: an entry in the output section and the new token.
• internal/skill/SKILL.md: a bullet and the command synopses.