refactor: simplify agent discovery result handling

[ALX99]

Simplification

Removes the neverthrow Result abstraction from the subagent discovery flow and replaces it with direct local return shapes:

• discoverAgents() now returns { agents } | { error }.
• resolveAgent() now returns AgentConfig | undefined.
• spawn_agent keeps the same tool-boundary behavior by throwing the same registration and unknown-agent errors from index.ts.

Why the current design is overcomplicated

neverthrow was only used inside home/.pi/agent/extensions/subagents/agents.ts for two local call sites. The abstraction did not provide a meaningful extension point because both consumers immediately matched the value at the boundary and converted the result into normal control flow.

For this repository, the extra ok(...) / err(...) constructors and .match(...) calls added a second result-handling vocabulary without changing behavior or improving isolation.

Behavioral contract

The following behavior must remain unchanged:

• Subagent discovery still reads ~/.pi/agent/extensions/subagents/agents/*.md through getAgentDir().
• Unreadable agent directories still fail loud during extension registration.
• Empty or fully invalid agent directories still fail loud during extension registration.
• Per-file read or frontmatter parse failures are still skipped so one broken role does not hide the remaining usable roles.
• Agent entries are still sorted by name before being used in the tool schema and prompt guidance.
• Unknown agent_type values still throw from the pi tool boundary and include the requested name plus available agents.
• Depth limiting, subprocess execution, rendering, throttling, stderr handling, abort behavior, model lookup, tool schema construction, and returned child output are not changed.
• Public tool name, parameters, prompt guidance, and child process invocation are unchanged.

Before and after

Before:

• agents.ts imported ok, err, and Result from neverthrow.
• discoverAgents() returned Result<AgentConfig[], DiscoverError>.
• resolveAgent() returned Result<AgentConfig, { requested; available }>.
• index.ts used .match(...) twice to unwrap those local results.

After:

• agents.ts no longer imports neverthrow.
• discoverAgents() returns a local discriminated union, { agents } | { error }.
• resolveAgent() returns AgentConfig | undefined.
• index.ts uses normal if checks and throws the same error strings at the same boundary.

Evidence:

• Removed the third-party Result vocabulary from home/.pi/agent/extensions/subagents/agents.ts.
• Removed .match(...) control flow from home/.pi/agent/extensions/subagents/index.ts.
• Reduced the agent lookup path to a direct find(...) result instead of wrapping and immediately unwrapping it.
• Kept dependency files untouched because this PR is intentionally limited to behavior-preserving code simplification; removing neverthrow from package.json/package-lock.json can be done separately after local install validation.

Validation

Commands run:

• Not run locally in this automation environment.

Static checks performed by inspection:

• Verified the only repository code references to neverthrow were in home/.pi/agent/extensions/subagents/agents.ts, package.json, and package-lock.json.
• Verified discoverAgents() still preserves all existing success and failure branches.
• Verified resolveAgent() still uses the same exact name comparison against the same sorted agents array.
• Verified index.ts still throws through the pi tool boundary for discovery failures and unknown agent_type values.

Expected validation for reviewer/local machine:

• cd home/.pi/agent/extensions && npm run check

Risk assessment

Behavior-sensitive areas examined:

• Extension registration: still throws before tool registration when discovery fails.
• Unknown agent errors: still include the requested agent type and formatted available-agent list.
• Agent discovery ordering: unchanged localeCompare sort remains in place.
• Broken individual agent files: still skipped, not fatal.

The change is safe because it only removes a local result wrapper and leaves all IO, parsing, sorting, schema generation, execution, and rendering behavior unchanged.

Scope

Intentionally not simplified:

• package.json / package-lock.json dependency removal for neverthrow, because lockfile changes should be generated and validated with npm install or equivalent local tooling.
• zod schemas, because they validate frontmatter and child process JSON at trust boundaries.
• Shell/profile configuration, because behavior preservation could not be proven safely without local shell validation.