feat(viking): tool filtering, error diagnostics, and URI-prefix instructions

[Million-mo]

Summary

Improves the Viking knowledge-graph capability in four ways:

1. Multi-prefix search (bug fix)

viking_search/viking_find now accept a list of target_uri prefixes. When
target_uri is omitted and an allowlist is configured, both tools pass every
allowed prefix to the SDK — previously they scoped to only the first allowed
prefix, silently dropping results from the other allowed trees.

2. Dynamic URI-prefix instructions

get_instructions() renders an Allowed URI Prefixes section when
allowed_uri_prefixes is configured, so the model sees the scoping boundary
up front and can pass the most specific target_uri directly, skipping
discovery probing (viking_ls) and the slower whole-allowlist search.

3. Diagnosable error returns

All 15 viking_* tools now include the exception type in error returns:
viking_search error (ReadTimeout): .... Empty-message failures like
httpx.ReadTimeout('') (slow semantic search on large resource trees) previously
rendered as a useless viking_search error: with no clue what went wrong.

4. Tool filtering

VikingCapabilityConfig gains enabled_tools / disabled_tools
(mutually exclusive, mirroring StdioMCPServerConfig). Operators can disable slow
semantic-search tools while keeping the deterministic ones:

capabilities:
  - type: viking
    mode: retrieve
    disabled_tools: ["viking_search", "viking_find"]

The filter applies in build_tools() after mode-based assembly and propagates
to both the toolset and the OpenCode /experimental/tool listing.

Tests

• 498 passed, 1 skipped in tests/capabilities/viking/
• New tests: multi-prefix defaults, enabled/disabled tool filtering, mutual-exclusivity
  validation, empty-message error diagnostics
• ruff check and mypy clean on changed source

Changelog

• 2026-08-18-viking-allowed-uri-prefixes.md (extended: search scoping behavior)
• 2026-08-19-viking-tool-error-diagnostics.md (new)
• 2026-08-19-viking-tool-filtering.md (new)