feat(devin): add subagents adapter (.devin/agents/{name}/AGENT.md) - #1902

Summary

Implements the subagents part of #1891: a native Devin Local custom subagent adapter.

Devin Local discovers custom subagent profiles as AGENT.md files in a directory-per-agent layout, where the directory name is the profile id:

• Project: .devin/agents/<name>/AGENT.md
• Global: ~/.config/devin/agents/<name>/AGENT.md

The adapter maps the rulesync name/description to top-level frontmatter and packs the Devin-specific fields (model, allowed-tools, permissions with allow/deny/ask, max-nesting) into a devin: section, round-tripping on import.

Evidence (verified upstream doc): https://docs.devin.ai/cli/subagents

Changes

• src/constants/devin-paths.ts: add DEVIN_AGENTS_DIR_PATH (.devin/agents) and DEVIN_GLOBAL_AGENTS_DIR_PATH (.config/devin/agents, resolved under the home root in global mode — note this is NOT .devin/agents/ under home).
• src/features/subagents/devin-subagent.ts: new DevinSubagent adapter (modeled on factorydroid-subagent.ts + the directory-per-agent layout from deepagents-subagent.ts).
• src/features/subagents/devin-subagent.test.ts: unit tests (paths, fromFile, fromRulesyncSubagent, toRulesyncSubagent, forDeletion, targeting, Devin-specific fields, global mode).
• src/features/subagents/subagents-processor.ts: wire DevinSubagent into the factory map and tool-target tuple. The filePattern is */AGENT.md so the directory-per-agent layout is discovered (the flat agents root is not scanned).
• src/features/subagents/subagents-processor.test.ts: add devin to the expected target lists.
• src/cli/commands/gitignore-entries.ts + .gitignore: add **/.devin/agents/ (project; the global ~/.config/devin/agents/ lives under home and is not gitignored at the project level).
• src/e2e/e2e-subagents.spec.ts: add Devin happy-path cases to the generate, orphan-deletion, import, and global-mode matrices.
• README.md, docs/reference/supported-tools.md, docs/reference/file-formats.md (+ auto-synced skills/rulesync/): flip the Devin subagents cell to ✅ and document the format/frontmatter.

Notable deviations from the precedent

• Directory-per-agent layout: output is .devin/agents/<name>/AGENT.md (not flat <name>.md). The subagent name is derived from the rulesync file name on generation and from the parent directory on import.
• Different global root: global mode emits to ~/.config/devin/agents/ rather than .devin/agents/ under the home directory, so getSettablePaths({ global }) returns a distinct relative dir for global vs project.

Deferred: permissions

The permissions part of the issue is intentionally deferred, matching the issue's own gating ("Evaluate a devin-permissions.ts adapter once the persistent permissions config file/path is confirmed"). Devin documents only the runtime "Always Allow" grant UX, with no confirmed static on-disk permissions config file/path. Once an official persistent permissions file/path is documented, a devin-permissions.ts adapter can follow up. Closes #1891 is appropriate since the actionable scope (subagents) is delivered and the permissions piece is blocked on an unmet upstream precondition.

Verification

pnpm cicheck is fully green (code + content): 270 test files / 6467 tests passing, formatting/lint/typecheck clean, cspell and secretlint clean, skills/rulesync docs in sync.

Closes #1891

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