feat(config): add .gittensory.yml surface to enable/scope repo-doc generation

[JSONbored]

Context

Part of the repo-specific CLAUDE.md/AGENT.md generation roadmap. Per the epic's design principles this feature is opt-in per repo, so it needs a config-as-code surface consistent with the existing .gittensory.yml settings/gate/review override layer. This issue adds the enable/scope/exclusion knobs; it does not implement generation itself.

Goal

Add a .gittensory.yml section that turns repo-doc generation on, scopes which file types it covers, and defines exclusion rules.

Requirements

• New config section (e.g. under a docs-generation or repo-docs key) following the existing precedence rules for this repo's config-as-code layer: yml overrides DB overrides defaults.
• Explicit boolean to enable the feature per repo, defaulting to disabled.
• Scoping option for which generated file types are in play (CLAUDE.md/AGENT.md only, vs. also skill files once the skill-generation issue lands).
• An explicit opt-in flag required before the engine will touch a repo that already has a hand-maintained CLAUDE.md/AGENT.md -- absent that flag, an existing file is left alone and generation is skipped rather than proposing an overwrite.
• Config schema changes include the corresponding resolver update and any generated-artifact regeneration this repo's process requires for schema/config changes.

Deliverables

• .gittensory.yml schema addition with resolver wiring and default-disabled behavior
• Exclusion-rule handling for repos with an existing hand-maintained CLAUDE.md/AGENT.md, tested for both the opted-in and not-opted-in cases
• Documentation of the new config keys alongside the existing config-as-code reference

Expected outcomes

• A repo with no docs-generation config never has this feature invoked against it
• A repo with an existing hand-maintained CLAUDE.md/AGENT.md and no explicit overwrite opt-in never receives a generated-overwrite PR

Part of the repo-doc generation roadmap: #2993

[linear-code]

JSO-2561