docs(kalshi): document Edge Connect teardown in the runbook - #200

`docsfetch` cut every MCP-served docs page at `MaxPageBytes = 10000`.
Measured against `malbeclabs/docs@751d45b`, that truncates **12 of the
27 MCP-reachable slugs**:

| bytes | page | | bytes | page |
|---|---|---|---|---|
| 33,856 | contribute-provisioning | | 12,200 | Swapping-sol-to-2z |
| 21,551 | troubleshooting | | 11,789 | geolocation |
| 17,551 | contribute | | 11,693 | glossary |
| 14,975 | contribute-ops-management | | 11,086 | kalshi-runbook |
| 13,694 | contribute-operations | | 10,880 | setup |
| 12,397 | kalshi | | 10,648 | multicast-admin |

The bound arrived in #770 as a defensive constant, not a measured one.
It was also silent — a bare `... (truncated)` appended to the content
string and no signal anywhere else — which is why this went unnoticed.
For `get_onboarding_runbook` it means an agent walks a user through a
procedure with the tail of the runbook missing.

Raises the bound to 64 KiB (~2x the largest real page), makes the marker
name the byte count and the source URL so an agent can point the user at
the rest, and reports `truncated` on both tool outputs.

Three things worth a reviewer's attention:

**The chat agent had its own copy of the fetch**, still bounded at
10,000 with the old silent marker, so the raised bound would not have
reached web or Slack users. `readDocs` in
`agent/pkg/workflow/v3/pipeline.go` now calls `docsfetch.Client.Read` —
a net −55 lines, and it picks up two things it never had: the
`ValidPage` slug guard (it previously interpolated the page straight
into a raw.githubusercontent URL) and a request timeout.

**A truncated runbook index is a hard error, not a short catalog.**
`loadRunbookCatalog` parses `runbooks.md` into the list of known
services; a cut index silently drops entries, and
`get_onboarding_runbook` then answers `unknown runbook "kalshi"` for a
runbook that exists. That is a confident wrong answer rather than a
degraded one, so it fails instead. It is the only `Read` call site whose
result the handler branches on.

**All three truncation cases now log at WARN.** The in-band marker
reaches the model but not an operator; the index refusal takes the tool
down for every caller, so it is the one that most needed a line.

`read_docs` also advertises three pages that no longer exist and one
that is an HTML redirect stub — in both the MCP and agent tool
descriptions. Real, but a separate change.

## Testing Verification

`TestRead_LargestRealPageIsNotTruncated` pins a 34,000-byte page reading
back whole; red-checked against the old constant, where it fails. The
marker's wording is asserted in `docsfetch`, which runs without
containers.

No test here can catch a docs page crossing the bound — nothing in lake
knows those sizes, they are fetched at runtime from another repo. Loud
truncation is the mitigation chosen over a cross-repo CI size check.

Two gaps, both environmental: the four new `api/handlers` tests and the
eval suite were **not executed locally**, since both packages'
`TestMain` starts containers and Docker was unavailable. Both compile
and vet clean. No eval references `read_docs`, so the agent change is
unexercised by that suite either way; `agent/pkg/workflow/v3` unit tests
pass.

`make lint` and `go build ./...` fail identically on `main`
(`dev/controlcenter` needs a built UI bundle). Both clean when scoped to
the touched packages.

## Blocks

malbeclabs/docs#200 — its new `## Teardown` section lands past the old
cut on `kalshi-runbook.md` (11,086 bytes). The alternative offered
there, trimming the page under 10,000, is not the fix taken.