New gh skill list to inventory installed agent skills - #13418

cli/cli#13245 is being closed by cli/cli#13418 (Tommy/skills list), which
adds `gh skill list` with the canonical shape:

  { skillName, hosts[], scope, sourceURL, version, pinned, path }

Where scope ∈ {project, user, custom}, hosts is always an array (empty for
--dir / custom scans, multi-valued for shared install dirs like
.agents/skills), and sourceURL is a full GitHub URL or local filesystem
path. The PR is still open, so this change keeps SkillView's tolerant
parser tolerant — upstream-canonical keys take priority but every legacy /
alternate key our adapter previously recognized stays as a fallback.

Adapter changes:
- GhSkillListRecord adds SourceUrl (string) and Hosts (ImmutableArray) as
  first-class fields; the existing Agents property is now an alias of
  Hosts so any call site that references it continues to work.
- ReadRecord reads `skillName` → fallback `name`/`skill_name`; `hosts` →
  fallback `agents`; `sourceURL` → fallback `repo`/`repository`. New
  ReadStringArray helper deduplicates the loop body.
- CapabilityProbeParser re-enables the `list` probe with the actual
  upstream flag set (`--json --agent --scope --dir`). When the gh binary
  doesn't ship the subcommand the probe is a no-op — ListSubcommandPresent
  stays false and SkillView's filesystem scan stays in charge.
- CapabilityProfile.SupportsListDir lights up automatically once the gh
  binary exposes the flag — unused today but available to any future tab
  that wants to scan a custom dir via gh.

Merge changes:
- CliList-only InstalledSkill records propagate SourceUrl into
  FrontMatter.Upstream so detail panes can render a clickable source link
  for skills that gh knows about but the filesystem scan hasn't seen.
- Agent membership is built from Hosts (upstream-canonical) instead of
  Agents; the legacy singular `agent` field still seeds a single
  membership when no array is present.

Tests:
- GhSkillListAdapterTests adds canonical-payload, multi-host, custom-dir,
  and precedence cases (hosts beats agents, skillName beats name).
- CapabilityProbeParserTests asserts the list-probe flag set instead of
  asserting the subcommand absent.

393/393 unit + 1/1 integration green. Nothing changes behaviorally on a
gh that doesn't ship `gh skill list` yet — the alignment lands now so when
PR #13418 merges, SkillView picks the canonical fields up automatically.