#15843: Docs(ja-JP): add channels overview and agent runtime

#### Summary Add Japanese (ja-JP) documentation for two key pages: - `docs/ja-JP/channels/index.md` — Chat channels overview (supported platforms list) - `docs/ja-JP/concepts/agent.md` — Agent runtime (workspace, sessions, skills) Translation style: natural tech Japanese tone (である体, minimal katakana, concise). #### Behavior Changes - No code changes - Docs only (new files added) #### Codebase and GitHub Search - [x] Searched for existing ja-JP docs — only 3 pages existed (`index.md`, `start/getting-started.md`, `start/wizard.md`) - [x] Checked i18n pipeline docs at `docs/.i18n/README.md` #### Tests - [x] `npm run format:docs:check` — passed - No code tests required (documentation only) #### Manual Testing N/A (documentation only) #### Evidence New files created: - `docs/ja-JP/channels/index.md` (47 lines) - `docs/ja-JP/concepts/agent.md` (106 lines) **Sign-Off** - Models used: Claude Opus 4.5 - Submitter effort: AI-assisted with human review - lobster-biscuit <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adds two new Japanese (ja-JP) documentation pages mirroring existing English docs: a Chat Channels overview (`docs/ja-JP/channels/index.md`) and an Agent Runtime overview (`docs/ja-JP/concepts/agent.md`). The content is a straightforward translation of the corresponding English pages and uses Mintlify-style frontmatter and root-relative internal links, fitting into the existing `docs/ja-JP/**` structure. <h3>Confidence Score: 4/5</h3> - This PR is low-risk and mostly safe to merge once minor docs guideline issues are addressed. - Changes are docs-only and largely mirror existing English pages with correct Mintlify frontmatter and internal link style. The only concrete issue found is an apparent mismatch with the repo guideline about emojis in docs, which is easy to fix. - docs/ja-JP/concepts/agent.md _{Last reviewed commit: 8e4207d} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->