feat(launch): make Headroom opt-in and wire RTK when the entry file lacks the block - #22

What a bare scc launch does, and does not, is now settled by one question: does this leave anything behind after the session ends?

Headroom moves behind --headroom

headroom wrap registers MCP servers into the agent's own config, and those registrations outlive the session that made them — which is why Headroom ships unwrap at all. What it buys is one session of compression. That trade is no longer made on the user's behalf: without --headroom, scc does not even look for the binary, and the report omits the headroom field entirely rather than saying skipped — a launch that never considered Headroom and one that considered it and could not use it are different answers.

Any --headroom-* flag implies --headroom, because accepting a flag and then ignoring it is how somebody spends a session believing they configured something. With the flag, the internal defaults are unchanged: --headroom-mcp none and --no-context-tool.

--no-headroom is gone; it has nothing left to turn off.

RTK goes the other way, bounded by the block

scc launch now makes sure the entry file the agent is about to load actually mentions RTK — the same setup scc rtk does standalone. The trigger is the block, not the binary: it runs when an entry file carries none, and does nothing at all when they all do.

That bound is what lets it be the default. It fires once per workspace rather than once per session, so a wired workspace pays no cargo prompt and no edit at the top of every session. The write is the point — an agent that never read the block never types the prefix, so installing the binary and leaving the file alone would buy nothing.

It splices with keep, so a block already there is left exactly as it is even when scc ships a different one. Replacing somebody's block is a real trade-off, and scc rtk is where it is made deliberately, not a side effect of starting an agent. --no-rtk opts out.

Two smaller consequences

• A plan-only run no longer writes. check now goes through to applyRTK, which closes a real hole: --rtk --dry-run used to splice the entry file — the one flag nobody expects to change anything doing exactly that.
• Block present, binary absent gets one warning and no action. The file tells the agent to use a binary this machine lacks, but that is a PATH problem on the user's side rather than something to fix mid-launch.

Tests

--no-headroom dropped from 18 incidental call sites, --headroom added to the ones that assert on the wrap. TestLaunchRTKIsOptIn gave way to five tests: writes when the block is missing, leaves a block that is already there byte-identical, --no-rtk omits everything, plan-only writes nothing, and it degrades without cargo.

gofmt, go vet and the full suite are green locally (the race detector needs a C compiler this machine does not have; CI covers it).

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https://claude.ai/code/session_018CAEQdLsrScdfaa1ut2H7g