A doctor says what would stop a run

[VeryComplexAndLongName]

Second of the six. Based on proposals-simpler-for-the-user (#417),
not on main — the change it implements is proposed there, and the
quality workflow only runs on pull requests targeting main, so this
gets its checks when #417 merges and this retargets itself. Run locally
in the meantime: npm run verify exit 0, 2015 tests across 159 files.

resolveChainStart already resolves every precondition of a run before
anything is spent — and only when somebody names a change and asks to
run it. Setting this up on a new machine means finding out what is
missing by being refused, which is the shape a-lease-says-who rejected
for the workspace lease.

openspec-ui-cli doctor asks directly: the runtime against the pinned
engines, the openspec CLI, which agents are installed, whether the
harness configuration reads, who holds the workspace, whether a git
identity is configured. Each finding says whether it stops a run or is
only worth knowing, and carries a remedy where one exists.
--change <id> adds the preflight's own answer for one change.

Nothing decides anything twice. Agent presence comes from
detectAvailableAgentsDetailed — the function the REST route, the VS
Code bridge and the agent picker already read — and openspec/npm go
through that same probe, exported as detectExecutable, rather than a
second one written beside it. This is a correction to the proposal made
while reading the code: it originally said a binary must be resolved and
never executed, which would have made this command answer "is this agent
here" differently from the picker in the same build.

A held workspace exits 0. Being busy is not being broken, and lease
already answers that way.

Live evidence, quoted in full in the tasks:

• This repository: Nothing here would stop a run., exit 0.
• PATH cut back to Node and the system directories: four findings —
  Node v24.18.0 against >=22 <23, npm 12.0.1 against >=10 <11, no
  openspec, no agent — exit 1.
• While a real openspec-ui-cli run held a scratch workspace: the
  holder reported with its pid and git author under "Worth knowing",
  exit 0.
• doctor --change two-steps-to-a-run: the preflight's own refusal and
  the setting that governs this is autonomyLevel, exit 1.

The range comparator is deliberately small: it reads the clause forms
this repository pins, on the major version, and reports a range it
cannot read as exactly that rather than treating it as satisfied.

🤖 Generated with Claude Code

[VeryComplexAndLongName]

Reopening to trigger the workflow now the base is main.