[JacobStephens2]

Summary

A new command, thirdshift setup. This PR adds only its no-terminal form: with no User config, it writes ~/.thirdshift/config.toml with every setting at its default and asks no questions.

 main
   args::parse
+    "setup"          → Command::Setup      # an argument after it → exit 2
+    "setup" <extra>  → argument error
+  Command::Setup → config::setup()
+    User config exists → parse it as a Run would (exit 1 if broken), keep it
+    none               → write DEFAULTS, "wrote the User config <path>" on stderr

The file it writes (the README's User config section now shows it too):

[merge]
always = false   # every Run is a Merge run, without the merge word; default false

[launch]
pull = false     # every Run first fast-forwards your checkout of the Base branch; default false

[email]
always = false                  # every Run sends a Run notification, without the email word; default false
# to = "you@example.com"        # where email goes when the command names no address; no default
from = "onboarding@resend.dev"  # the sender; default onboarding@resend.dev, which only delivers to your Resend account's address

[logs]
dir = "~/.thirdshift/logs"   # where session logs go; default ~/.thirdshift/logs

help now lists thirdshift setup.

Evidence

• Before: thirdshift setup was read as an Issue URL and failed with an argument error.
  After: tests/setup.rs has 7 tests, all passing. They run the binary through the piped Scenario harness and check:
  • the file is written, exit 0, stdout empty
  • every key has a comment giving its default, and only email.to is commented out
  • a Run on the written file behaves as with no User config: ready for review, logs in ~/.thirdshift/logs
  • --email still stops for want of email.to
  • setup extra exits 2 with the help text and writes nothing
  • an existing User config keeps its values and comments
  • a broken one is refused and left untouched
• A unit test checks that DEFAULTS parses to the same UserConfig a Run uses with no file.
• The full cargo test suite passes, and clippy is clean.

Merge Danger

Door: two-way

It adds a command and writes a file only when asked to. Nothing changes for Runs.

Blast Radius: minimal

Only people who run thirdshift setup are affected.

Unaddressed findings

Standards

• Duplicated Code / Shotgun Surgery: the default values appear in DEFAULTS, UserConfig::defaults, email::DEFAULT_FROM, the README and the help text. The spec requires the README to show the file, and a unit test ties DEFAULTS to UserConfig::defaults. The README copy is documentation, not code.
• Duplicated Code: setup() repeats load()'s "read, else NotFound" shape. It is two short matches whose arms differ, and a shared helper wouldn't make either clearer.
• email.from written out explicitly pins onboarding@resend.dev. The spec requires it ("email.from = "onboarding@resend.dev"", and real values wherever there is a default).
• "Setup with no terminal" doc comment is speculative: the interactive form is planned in thirdshift setup #126, so the comment names the case this slice covers.
• DEFAULT_FROM is pub rather than pub(crate): this is a binary crate, and the rest of the codebase uses pub.
• AGENT_OPENS_PR/PR_URL are copied into tests/setup.rs: this matches the existing practice in five other test files. Moving them into tests/support belongs in its own change.
• Squash the interrupted-run WIP commit: I left the history as it is, since the branch had to build on that commit rather than rewrite it.

Spec

• No terminal check: setup writes the defaults even from a terminal. The questions don't exist yet, so a terminal check would have nothing to branch to. It belongs to the thirdshift setup #126 slice that adds the questions.
• Existing User config handling is beyond Setup: thirdshift setup writes the defaults User config with no terminal #127: I kept it because it follows thirdshift setup #126 stories 32 and 34, and it never overwrites a user's file. It keeps values and refuses a broken file, and its test and README now say "keeps its values" rather than "left as it is", so the next slice can add missing keys without reversing anything.

Closes #127

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