Add: high-level design with Mermaid diagrams, linked from the README

[0xLeif]

Summary

• Adds docs/HLD.md, a high-level design of rune written against the code on main (1f3372d). It covers purpose, context, components, the three execution models (run, watch, session), key flows (dispatch, run, watch, session start, send and settle, read and list, attach, stop and archive), session internals (lifecycle, the supervisor's event loop, the PendingSend settle decision, the control protocol, transcript bounds), on-disk data, runtime and release, security and trust boundaries, failure modes with a timeouts table, decisions and a glossary.
• 17 Mermaid diagrams: 8 flowcharts, 6 sequence diagrams, 2 state diagrams and 1 ER diagram. Each renders with mermaid-cli.
• README: the existing "Architecture & Internals" section gets a short paragraph, one overview diagram and a link to the HLD. The existing bullets are unchanged.
• The HLD links to the existing guides instead of repeating them: getting started, sessions, PTY architecture, releasing. It also links the specs, ROADMAP.md, CHANGELOG.md, the SpecSync change records and harnesses/.

Notes for review

• Finding: the supervisor is not detached by setsid. start spawns the supervisor with pgroup: true. That makes it a process-group leader, so its own Process.setsid call is refused (EPERM, and the code rescues it). I measured this on macOS for this PR: I started a real rune session start -- sleep 120 from bash inside a PTY, then closed the PTY and sent SIGHUP to the shell. The supervisor kept the shell's session id, led its own process group, and survived. The HLD states this and marks Linux and other shells as Unknown. Two places describe setsid as the mechanism: the comment on Supervisor#detach_from_terminal and the AGENTS.md line "detached (Process.setsid)". No code is changed here.
• No SpecSync change record. docs/ and README.md are not in meaningful_paths, and no active change lists README.md, so nothing goes stale. I deliberately left docs/sessions.md, ROADMAP.md, specs/ and docs/i18n/ alone, because they appear in accepted changes' affected paths.
• Translations. The nine docs/i18n/README.*.md files do not get the new section. The README says the English file is authoritative.
• hi. hi/ and INTENT.md already exist. hi check passes (155 criteria, 11 families), and no criteria changed.
• Pages. The repo has no GitHub Pages site. GitHub renders the Mermaid fences natively.

Test Plan

• All 17 HLD diagrams and the README diagram render with bunx -p @mermaid-js/mermaid-cli mmdc
• Every relative link and anchor in docs/HLD.md and README.md resolves
• fledge run docs-check passes
• fledge lanes run verify passes (642 examples, 0 failures, spec coverage 100%)
• fledge trust verify passes on the committed range
• hi check passes
• CI is green on this PR

🤖 Generated with Claude Code

https://claude.ai/code/session_01V3ZZAEiUP7xRJPozhZb6rL