docs: changelog entry for the daily spend rail (#1546)

[santoshkumarradha]

Closes #1546

What

The daily spend rail fix is already on dev via #1627 (commit ac84606f4): interactive turns enforce the day's budget, task workers and errands are exempt, and the tests are there too (TestSpendRailRefusesTurnWhenDailyBudgetExceeded, TestBudgetScopeAndLiveConversationRecovery). The takeover branch (fix/1546-daily-spend-rail) turned out to be a strict duplicate with older semantics — it was never pushed. The one piece that never landed is this changelog entry, which is all this PR adds.

The entry

• kind: fixed, surfaces [chat, engine], pr: 1546.
• States the old behavior (only per-conversation limits were checked), the new gate, and the exact door the refusal names — verified against the shipped string in internal/session/rail.go: daily limit reached · %s spent of %s · /budget day changes it.
• Notes the task-worker/errand exemption with its rationale, and that the budget is read fresh each turn.

Gates

Docs-only diff: no video owed under the bar. No workloads ran locally (raw-ssh-Spark-only rule honored; none needed — GitHub CI validates). Three review passes are posted as separate comments. PR stays draft/[WIP] until CI is green; never merged by its author.

—
Drafted with CodeAF · reviewed and owned by the author

[santoshkumarradha]

Pass 1 — ARCHITECTURE

Checked: where the change sits and how it is shaped.

• One new file, docs/changes/unreleased/1546-daily-spend-rail.md; no code, no wiring, no second convention introduced.
• House changelog format followed exactly (front matter kind/title/pr/surface/invalidates, then prose), matching the sibling entries in the directory.
• The pr: field matches the filename's number, as sibling entries do; surface: [chat, engine] matches where the behavior lives — the gate is engine-side (internal/session), the refusal is what a chat turn sees.
• The change documents behavior merged via Consolidate verified CodeAF task, chat and runtime fixes #1627 rather than re-implementing it in the docs tree — the right layer for a paper-trail gap.

Found: nothing to change.

_{drafted with CodeAF}

[santoshkumarradha]

Pass 2 — SECURITY

Checked: what the entry exposes and what it touches.

• Docs-only diff: no code paths, no secrets, env vars, credentials, endpoints or internal hosts named; nothing untrusted reaches anything.
• The quoted refusal string contains no user data — it matches the shipped format string in internal/session/rail.go (daily limit reached · %s spent of %s · /budget day changes it), where the figures are filled at runtime and never persisted here.
• The entry describes public behavior only; it reveals nothing about the machine, the ledger's storage, or configuration values.

Found: nothing to change.

_{drafted with CodeAF}

[santoshkumarradha]

Pass 3 — CODE EFFICIENCY & DESIGN

Checked: economy of the wording and truth of every claim.

• The invalidates block carries the old behavior, the gate and the door in one sentence; the prose adds only the exemption rationale and the freshness note — no padding.
• No duplication: no other docs/changes entry mentions the daily budget or spend rail (grep-verified against dev), so this is the single line of truth for the change.
• Claims verified against the code on dev: the gate is >= ("reaches"), the spend is the larger of the ledger's day and the crew's held day (max), task workers and errands are exempt, and the refusal names /budget day exactly.

Found: nothing to change.

_{drafted with CodeAF}

[santoshkumarradha]

@ZeroPoint95 ready for review: all three review passes completed with no actionable findings, and final-head CI is green. This is a docs-only changelog change.