docs: remove the reference prose that restates generated output

[AbysmalBiscuit]

docs/configuration.md and docs/commands.md are hand-maintained copies of output devkit already generates, and both are documented as losing when they disagree with it. Delete the redundant prose and point at the generated surfaces.

Why

• docs/configuration.md is ~9.5k words of per-key reference. Those descriptions already exist as Rust doc comments, and again as the 156 descriptions devkit schema renders from them. Three copies of one meaning.
• docs/commands.md is ~7.2k words of per-subcommand flags and gates, restating -h.
• AGENTS.md already tells agents to dump devkit schema rather than trust a reference doc. skills/using-devkit/SKILL.md already says the -h wins when the two disagree. Both files are on record as the lower authority.
• skills/using-devkit/, ~6k words, is the only one of the three holding what nothing generates: when to reach for a command, what a denial means, which gotcha bites. It is also the one that gets read.

The sync is manual and it rots silently. Recent example: #57 updated docs/configuration.md for a new task run shape and left skills/using-devkit/references/tasks.md stale.

Scope

1. Cut the per-key prose from docs/configuration.md and the per-flag prose from docs/commands.md. Point both at devkit schema and -h.
2. Move the surviving rationale into AGENTS.md, which already holds the invariants.
3. Move the surviving gotchas into skills/using-devkit/references/, where an agent will load them.
4. Keep README.md and docs/install.md. Someone evaluating devkit on GitHub cannot load a skill.
5. Retarget every link and error message pointing at a docs/configuration.md#... anchor.

Nothing here should delete a fact that exists in only one place. Each cut line is either generated elsewhere or relocated by steps 2 and 3.

Related

#58 makes the doc comment the single source of truth by moving examples there and covering them with doctests. That is the replacement this issue clears room for; either can land first, but doing #58 first means less prose to relocate here.

Open questions

• Does devkit schema init's commented starter become the authoring path for a new devkit.toml, replacing docs/configuration.md outright?
• Does docs/commands.md survive as a thin index of subcommands, or go entirely?

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