feat(sdk/go): expose target_uri skill scoping (parity with #2813)

[r266-tech]

What

#2813 promoted account-shared skills (viking://agent/skills) to a first-class, scoped capability and shipped a target_uri parameter across:

• the Python SDK — all 7 skill methods (add_skill/list_skills/find_skills/validate_skill/get_skill/update_skill/delete_skill)
• the Rust CLI (crates/ov_cli)
• the server skills router — every endpoint accepts target_uri

The Go SDK was the only client left without it. Today Go adopters cannot scope skill operations to user-vs-agent roots and have to drop down to Python/CLI to reach account-shared skills.

This PR brings the Go SDK to parity.

Change

Adds an optional TargetURI any field to the six skill option structs plus a new DeleteSkillOptions, and threads target_uri through every skill method matching the Python SDK + server placement exactly:

Method	Transport
ListSkills, GetSkill, DeleteSkill	query param
AddSkill, FindSkills, ValidateSkill, UpdateSkill	JSON body

target_uri is sent only when set (a nil TargetURI omits it), mirroring the Python SDK's if target_uri is not None semantics and the server's default-root fallback (_agent_skills_root treats absent/empty as the default root). The field type matches the existing TargetURI any convention already used by FindOptions/SearchOptions in types.go.

Note: unlike retrieval Find/Search (which client-side normalize via normalizeTarget), the skills methods send target_uri as-is and let the server resolve it — this matches the Python SDK in #2813 exactly (skill methods pass target_uri raw; only find/search call _normalize_target_uri).

DeleteSkill previously took no options. To carry the scope without breaking existing DeleteSkill(ctx, name) callers, it now takes a variadic opts ...*DeleteSkillOptions (only the first value is used); the other six methods gain TargetURI as a plain additive struct field. So the change is source-compatible — no existing call site needs to change.

Verification

• New TestSkillRequestsScopeTargetURI asserts query-vs-body placement of target_uri for all 7 methods.
• New TestSkillRequestsOmitTargetURIWhenUnset asserts target_uri is omitted (no body key / no query param) for all 7 methods when unset, so existing callers are unaffected.
• go test ./... and go vet ./... pass; gofmt clean.

Per-method transport (query vs body) was verified against the server router so the chosen location is the one the server actually reads, not an assumption — openviking/server/routers/skills.py:

• body: FindSkillsRequest.target_uri, ValidateSkillRequest.target_uri, UpdateSkillRequest.target_uri, and add reads it from the request body (matching Python add_skill's request_data)
• query: list_skills, get_skill, delete_skill take target_uri: Optional[str] = None as a query parameter

Field names were verified against live main (sdk/go/types.go, sdk/go/skills.go) and the merged #2813 Python diff — not guessed.