Add DSH (DeepSeek Harness) as an instructions-only install target

[yangwuan55]

What

Adds dsh as a new installer target for DSH (DeepSeek Harness), alongside claude / cursor / codex / opencode / hermes / gemini / antigravity / kiro / copilot-*.

codegraph install -t dsh            # or pick DSH in the interactive prompt

DSH agents have no MCP client. Instead they ingest plain-markdown instruction files and re-digest them every turn:

• local → <project-root>/AGENTS.md
• global → $DSH_HOME/AGENTS.md (defaults to ~/.dsh/AGENTS.md)

So this target is instructions-only: it writes the existing marker-fenced CodeGraph usage block (instructions-template.ts) and nothing else — no MCP server config anywhere. The agent drives codegraph explore / other subcommands through its shell tool, which prints the same output as the MCP tools. That is precisely the "non-MCP harness" audience the instructions block was reintroduced for in #704.

Why a new interface knob

AgentTarget grows one optional field:

/** Defaults to true; all existing targets are unchanged. */
readonly requiresRestart?: boolean;

DSH sets it to false because running sessions re-read their instructions file each turn — so the install/uninstall summaries now say "no restart needed" when every touched target is restart-free, instead of asking users to restart for nothing. Every existing target keeps today's behavior.

Details

• New file src/installer/targets/dsh.ts + registry entry + 'dsh' in the TargetId union; nothing else about the target system changes.
• Uninstall strips only the marker block and preserves surrounding user content; a block-only AGENTS.md is deleted entirely, matching removeMarkedSection semantics.
• Test-hygiene fix surfaced while testing on a machine that actually runs DSH: setHome() now clears $DSH_HOME at setup, not only at restore — otherwise an ambient value leaks the real ~/.dsh into every supposedly-isolated test.
• README badges + supported-agents list, CLI help text, CHANGELOG entry under Unreleased.
• No docsUrl claimed (didn't want to invent a URL; happy to add one if there's a canonical link.

Verification

RUN v2.1.9 /private/tmp/codegraph-pr

✓ tests/installer.test.ts (3 tests) 4ms
✓ tests/installer-targets.test.ts (244 tests | 3 skipped) 314ms

Test Files 2 passed (2)
Tests 244 passed | 3 skipped (247)
Start at 15:15:05
Duration 672ms (transform 159ms, setup 0ms, collect 245ms, tests 319ms, environment 0ms, prepare 114ms) → 244 passed, 3 skipped (pre-existing skips), including the generic per-target contract suite exercising at both locations (install / idempotency / uninstall-reverses / printConfig-no-writes), plus dedicated tests: override, sibling-content preservation across install/uninstall, no-MCP printConfig.

• clean.
• End-to-end sandbox run (redirected HOME/DSH_HOME): at both locations, byte-identical re-run, uninstall round-trip, mixed install still asking for a restart, unknown-target error now listing .

Happy to adjust naming () or scope — e.g. splitting the orchestrator message tweak into its own PR if reviewers prefer.
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