fix(buzz-agent): keep goose's bundled skills out of Buzz agents

[micspiral]

What

Stops Buzz agents advertising goose's two compiled-in skills, goose-doc-guide and web-search.

Stacked on #3262 (micn/buzz-agent-goose-core), not on main — the code it changes only exists on that branch.

Why

buzz-agent on main scanned four filesystem directories for skills and had no builtins at all (hints.rs:8, :204-217). The goose swap registers goose's skills extension by name, so the platform factory runs SkillsClient::new (platform_extensions/mod.rs:219-221) with builtins on. Neither builtin is a Buzz skill, and web-search (added upstream in goose 6d6abf74a, arrived here with the d1d8f45 pin bump) instructs the model to shell out to uvx ddgs / Tavily / SearXNG — a capability Buzz does not provide.

That is a scope widening as a side effect of a port that is supposed to be behaviour-preserving.

How

with_builtin_skills(false) is only reachable off the constructor, so buzz builds the client itself and registers it with ExtensionManager::add_client. The prompt index is filtered to match — a skill in the index but absent from the client is a dead load_skill reference.

Two things that route had to preserve:

• Bare tool name. is_unprefixed_extension (extension_manager.rs:392-400) keys off the ExtensionConfig, not the registration route. This passes the same ExtensionConfig::Platform { name: "skills" } the factory does, so the table's unprefixed_tools: true still applies and the model still sees load_skill, not skills__load_skill.
• Working directory. The context is given the session explicitly, because SkillsClient::new reads session.working_dir and falls back to the process cwd without it — which for a desktop-spawned agent is not the nest.

Measured, not reasoned

Real ~/.buzz nest, built binary, local fake SSE provider (no model call, no meter). Same probe against the a41b78f03 binary as baseline:

	a41b78f03	this branch
Skill index	13 skills	11 skills
Tool list	["load_skill"]	["load_skill"]
load_skill("buzz-cli")	body returned	body returned
load_skill("web-search")	body returned	Skill 'web-search' not found.
System prompt	12,632 B	12,079 B

Tests

tests/skills.rs gains assertions it was missing. The existing test checked only stopReason == end_turn plus a second round — a load_skill returning "Skill not found." passes both, so it could not tell a working skills path from a broken one. It now asserts the skill body arrived as a tool result.

The new test pins the builtins from both sides (absent from the index AND unresolvable through the tool); index-only would pass while the tool still served them. Verified to fail on the unpatched lib.rs, so the gate is known to reject.

buzz-agent: 111 unit + 36 integration green, clippy --all-targets -D warnings clean, fmt --check clean, at 0a4c78e1e.

Not in scope

~/.config/goose/skills is still scanned — an operator's personal goose skills apply to their own agents. .claude/skills was already scanned on main; the only new claude path is the global one, which is absent on both lab boxes. Hint loading is effectively unchanged. All three are deliberate, per direction in #feature-buzz-with-gdk.

[micspiral]

Closing — this should not have been a PR against a feature branch. The fix stays available as the single commit 0a4c78e1e on dario/gdk-skills-builtin-optout for whoever owns #3262 to cherry-pick.