fix(opencode): allow aft_outline + aft_zoom for all hidden subagents

Adds the read-only AFT navigation tools to historian, dreamer, and
sidekick allow-lists. These are exactly the right shape for hidden
subagents that need to inspect code structure without dragging in
whole files: aft_outline returns a symbol-level outline, aft_zoom
returns the source of a single named symbol.

Per-agent rationale:

  - historian / historian-editor / compressor: can now verify a
    referenced symbol or skim file structure when writing accurate
    compartment summaries, without falling back to whole-file reads.

  - dreamer: the key-files identification prompt at
    features/magic-context/key-files/identify-key-files.ts:257-258
    already tells the model to use aft_outline / aft_zoom. Without
    them in the allow-list those calls would have been denied,
    which would silently regress the key-files identification task.

  - sidekick: can pull lightweight symbol-scoped context when the
    user's prompt references a specific file or symbol. Sidekick's
    read still stays denied — it should pull symbol-scoped views
    via aft_zoom, not arbitrary file contents.

Allow-lists now:

  HISTORIAN_ALLOWED_TOOLS = read, aft_outline, aft_zoom
  DREAMER_ALLOWED_TOOLS   = read, grep, glob, bash, aft_outline,
                            aft_zoom, ctx_memory, ctx_search, ctx_note
  SIDEKICK_ALLOWED_TOOLS  = ctx_search, ctx_memory, aft_outline,
                            aft_zoom

Tests updated and expanded: 1479 pass / 0 fail (+3 new aft assertions
across the three agents). Permission integration shape tests now
verify the exact final ruleset for each agent.