[codex] Raise agent heartbeat concurrency default (#4954)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - Agent heartbeat settings control how much parallel work one employee
can run
> - The previous default of 5 concurrent runs was too restrictive for
active local agent teams
> - The shared default, heartbeat clamp, docs, and route/import/UI
expectations need to agree
> - This pull request raises the default heartbeat concurrency to 20
while keeping explicit headroom up to 50 for power users
> - The benefit is higher throughput for agent teams without each new
agent needing manual runtime config edits

## What Changed

- Raised `AGENT_DEFAULT_MAX_CONCURRENT_RUNS` from 5 to 20.
- Raised the heartbeat service max clamp from 10 to 50, keeping the new
default below the ceiling.
- Updated V1 implementation docs and tests that assert default
imported/exported runtime config.
- Updated the new-agent UI runtime config test to assert the shared
default constant instead of duplicating the numeric value.

## Verification

- `pnpm exec vitest run
server/src/__tests__/agent-permissions-routes.test.ts
server/src/__tests__/company-portability.test.ts
ui/src/lib/new-agent-runtime-config.test.ts`

## Risks

- Medium risk: new agents can consume more local execution capacity by
default. The heartbeat scheduler still respects configured max
concurrency and budget/pause controls, and operators can lower or raise
the per-agent cap within the `1..50` clamp.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

- OpenAI Codex, GPT-5 coding agent, tool use and local command
execution. Exact context window was not exposed in the runtime.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] If this change affects the UI, I have included before/after
screenshots
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

server/src/__tests__/agent-permissions-routes.test.ts

@@ -901,7 +901,7 @@ describe.sequential("agent permission routes", () => {
           heartbeat: {
             enabled: false,
             intervalSec: 3600,
-            maxConcurrentRuns: 5,
+            maxConcurrentRuns: 20,
           },
         },
       }),
@@ -939,7 +939,7 @@ describe.sequential("agent permission routes", () => {
           heartbeat: {
             enabled: false,
             intervalSec: 3600,
-            maxConcurrentRuns: 5,
+            maxConcurrentRuns: 20,
           },
         },
       }),

server/src/__tests__/company-portability.test.ts

@@ -2158,7 +2158,7 @@ describe("company portability", () => {
       runtimeConfig: {
         heartbeat: {
           enabled: false,
-          maxConcurrentRuns: 5,
+          maxConcurrentRuns: 20,
         },
       },
     });
@@ -2237,7 +2237,7 @@ describe("company portability", () => {
       runtimeConfig: {
         heartbeat: {
           enabled: false,
-          maxConcurrentRuns: 5,
+          maxConcurrentRuns: 20,
         },
       },
     }));

server/src/services/heartbeat.ts

@@ -145,7 +145,7 @@ const MAX_RUN_EVENT_PAYLOAD_OBJECT_KEYS = 100;
 const MAX_RUN_EVENT_PAYLOAD_DEPTH = 6;
 const HEARTBEAT_MAX_CONCURRENT_RUNS_DEFAULT = AGENT_DEFAULT_MAX_CONCURRENT_RUNS;
 const HEARTBEAT_MAX_CONCURRENT_RUNS_MIN = 1;
-const HEARTBEAT_MAX_CONCURRENT_RUNS_MAX = 10;
+const HEARTBEAT_MAX_CONCURRENT_RUNS_MAX = 50;
 const LIVENESS_BOOKKEEPING_ACTIVITY_ACTIONS = [
   "environment.lease_acquired",
   "environment.lease_released",