Support file-backed skill consumption mode for sandbox-mounted agents

[astrobot-houston]

Originally proposed by @rpenmatc04 in #466. Their description is reproduced verbatim below.

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Summary

• add an opt-in skillMode: "files" agent/profile setting
• advertise exact SKILL.md paths in the system prompt
• omit both activate_skill and read_skill_resource in files mode
• keep activate as the default for compatibility
• route packaged skills through the ordinary read tool

Why

Some coding-agent runtimes mount selected skill bundles into their sandbox filesystem. In that architecture, skills should behave exactly like local files: the model reads SKILL.md with its existing file tool and resolves bundle resources relative to the skill directory. A separate activation protocol adds an unnecessary tool surface and diverges from local behavior.

Verification

• pnpm --dir packages/runtime run build
• pnpm --dir packages/runtime run check:types
• focused Biome check over changed files
• pnpm --dir packages/runtime run test — 776 passed, 1 skipped

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Original implementation from #466 by @rpenmatc04