Route Functional Planner Jira reads through the executor and drop its terminal grant

## Context

The Functional Planner (`.github/agents/project-planning/functional-planner.agent.md`) declares a terminal grant. PR #2602 narrowed its prose to state that the terminal exists solely for `jira` skill CLI read commands, because that is the only Jira read path available: Jira has no tool family, only a CLI.

Two consequences remain:

1. The no-mutation commitment for Jira is a policy the agent holds, not a boundary the tool list enforces. Azure DevOps and GitHub mutation is genuinely unreachable because those grants are read-only tool families; Jira is not.
2. The Jira Backlog Executor's claim to be the only agent that can reach Jira is not strictly true while this grant exists.

## Why this was deferred

Provenance evidence reclassified the original finding. Both predecessor agents on `main` carried the same terminal grant, and the Jira predecessor enforced its read-only boundary in prose alone naming the same verbs. The consolidation inherited the pattern rather than introducing it, so PR #2602 corrected the claim instead of removing a long-standing capability.

Removing the grant without a replacement read path would break Jira PRD planning.

## Proposed work

Add a read-only dispatch from Functional Planner to the Jira Backlog Executor, then remove the terminal grant from Functional Planner.

## Acceptance criteria

- [ ] Functional Planner obtains Jira reads through a dispatch to the executor rather than by running the CLI itself
- [ ] The dispatch contract is read-only and rejects any mutating verb
- [ ] Functional Planner's frontmatter no longer grants terminal execution
- [ ] Its no-mutation statement becomes enforceable rather than policy, and the prose is updated to say so
- [ ] The Jira executor's only-agent-reaching-Jira claim becomes accurate

## Evidence

- PR #2602 review threads covering Functional Planner tool authority
- `.github/agents/project-planning/functional-planner.agent.md` — the terminal grant and its narrowed purpose statement
- `.github/agents/project-planning/subagents/jira-backlog-executor.agent.md` — the reachability claim